1 PANORAMA MONITORING DEVICE FOR TRAILER OF COMBINATION VEHICLE BACKGROUND OF INVENTION 1. Field of the Invention The present invention relates to a panorama monitoring device for trailer of combination vehicle, more particularly to a panorama monitoring device for trailer of combination vehicle, which can monitor front coupler status at all time, and which conducts shooting in corresponding direction in response to turning, braking or reversing of the combination vehicle, and the images are transmitted to a display unit for a driver to see clearly so as to make vehicle driving safer. 2. Brief Description of Prior Art As the number of cars are increasing, traffic accident emerges in endlessly. Among which, the traffic accident caused by combination vehicles happens from time to time. Current combination vehicle is defined by the combination of a front tractor truck and a rear trailer coupled together by a coupler. As the trailer do not have power, its turning and reversing must be done through the tractor truck. When the combination vehicle is turning or reversing, the driver mainly uses back mirror to grasp side or rear situations of the vehicle. However, as there 2 is container or box carried by the trailer, which has not only big volume but also considerable length, so it is easy to block the driver's sight line of observing side or rear status of the vehicle through the back mirror, and thus sight dead angle is generated. Accordingly, it often occurs that the combination vehicle, during its turning or reversing, has the danger of sideswiping another car or even running over pedestrian due to unawareness of people beside or behind caused by sight dead angle. Moreover, when the combination vehicle passes through road surface bumps, the coupler connecting the tractor truck and the trailer together will be subjected to action of vibration force. When the combination vehicle brakes, the trailer will displace forward due to the of inertia force, and hence the coupler will withstand the forward impact force applied by the trailer. When the combination vehicle starts to move from stagnation state, the coupler will bear the backward traction force applied from the trailer. These vibration force, forward impact force and backward traction force may cause damage to the coupler, and then may cause separation between the tractor truck and the trailer resulted from damage of the coupler. Accordingly, this may trigger traffic accident. In view of the above facts, the inventor of the present invention hereby proposes the present invention according to his new idea and improvements made on prior art, and based on his abundant experience of manufacturing and product development in relevant field, with a 3 purpose to achieve better implementation value. SUMMARY OF THE INVENTION The main object of the present invention is to provide a panorama monitoring device for trailer of combination vehicle, which can monitor the status of coupler in front of the trailer all the time, and which conducts shooting in corresponding direction in response to turning, braking or reversing action of combination vehicle, and the images are transmitted to a display unit for a driver to see clearly so as to make vehicle driving safer. In order to achieve above object, the panorama monitoring device for trailer of combination vehicle proposed by the inventor of the present invention is configured as below. The panorama monitoring device for trailer of combination vehicle mainly comprises: a combination vehicle having a front tractor truck and a rear trailer, which are coupled together by a coupler provided therebetween, the trailer having a left direction light, a right direction light, and a tail light assembled therewith; a camera unit including a front camera lens, a rear camera lens, a left camera lens and a right camera lens respectively installed on a front side, a rear side, a left side and a right side of the trailer; 4 a processing unit provided in the trailer, which is electrically connected with the left direction light, the right direction light, the tail light; and the front camera lens, the rear camera lens, the left camera lens and the right camera lens being coupled to the processing unit; a display unit provided in the trailer, which is electrically connected with the processing unit by a conductive wire. According to the panorama monitoring device for trailer of combination vehicle of the present invention, the trailer is a trailer for camping. According to the panorama monitoring device for trailer of combination vehicle of the present invention, the trailer is a cargo trailer. Configuring like this, the front lens takes picture of the coupler installed in front of the trailer and the images are transmitted from the processing unit through a conductive wire to a display unit for a driver to see the status of the coupler. When the driver makes turning, braking or reversing to trigger the left direction light, the right direction light or the tail light, the processing unit will drive the left camera lens, the right camera lens or the rear camera lens, and the images taken by the camera lenses are transmitted to the display unit for the driver to see if there is any sight dead angle present in turning, braking or reversing, so as to avoid sideswiping or running over foreign matters. Thus, this device makes vehicle driving safer.
5 BRIEF DESCRIPTION OF ACCOMPANYING DRAWING Figure 1 is a front view of the present invention; Figure 2 is a top view of the present invention; Figure 3 is a block diagram showing the circuit connection of the present invention; DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The objects, the technical contents and the expected effect of the present invention will become more apparent from the detailed description of a preferred embodiment in conjunction with the accompanying drawings. Firstly referring to Figures 1 to 3, the panorama monitoring device for trailer of combination vehicle of the present invention comprises a combination vehicle (1), a camera unit (2), a processing unit (3), a display unit (4) and a manual control unit (5). The combination vehicle (1) has a front tractor truck (11) and a rear trailer (12), which are coupled together by providing a coupler (13) therebetween. The trailer (12) has a left direction light (14), a right direction light (15), and a tail light (16) assembled therewith. The camera unit (2) includes a front camera lens (21), a rear camera lens (22), a left camera lens (23) and a right camera lens (24) 6 respectively installed correspondingly on a front side, a rear side, a left side and a right side of the trailer (12). The processing unit (3) is provided in the trailer (12) and is electrically connected with the left direction light (14), the right direction light (15) and the tail light (16); the front camera lens (21), the rear camera lens (22), the left camera lens (23) and the right camera lens (24) are also coupled to the processing unit (3). The display unit (4) is provided in the trailer (12), which is electrically connected with the processing unit (3) by a conductive wire. The manual control unit (5) is provided in the tractor truck (11), which is electrically connected to the processing unit (3) by a conductive wire. When in implementation with above configuration, the processing unit (3) sets the front camera lens (21) in normally opened state so as to shoot the coupler (13) located in front of the trailer (12), and the images taken by the front camera lens (21) are transmitted through a conductive wire to the display unit (4) for a driver seating in the tractor truck (11) to monitor the status of the coupler (13) all the time. When the coupler (13) is damaged or loosened due to vibration force caused by passing through road surface bumps, or due to forward impact force applied by the trailer caused by the action of inertia force which is generated by braking of the tractor truck, or due to backward traction force caused by starting of the combination vehicle (1) from stagnation state to move, the 7 driver of the combination vehicle can see clearly the image of the coupler (13) shown on the display unit (4) to grasp the status of the coupler (13) and to takes necessary disposal upon confirmation of damage or loosening of the coupler (13). Moreover, when the driver of the combination vehicle (1) makes a left turning in driving to actuate the lighting of the left direction light (14) provided on the trailer (12), the processing unit (3) connecting with the left direction light (14) simultaneously receive the actuation signal coming from the left direction light (14), and then the processing unit (3) drives the left camera lens (23) to shoot the left side zone of the trailer (12), and the images shot by the left camera lens (23) are sent back to the processing unit (3). Next, the processing unit (3) sends the images of the left side zone of the trailer (12) through a conductive wire to the display unit (4), and simultaneously turns off the images shot by the front camera lens (21) according to preset programming of the processing unit (3), so as to avoid the driver from distraction. In this manner, when the driver makes a left turning, he can see clearly the left side zone of the trailer (12) through the images shown on the display unit (4), so as to avoid the danger of sideswiping with the other car or running over foreign matter or pedestrian due to formation of sight dead angle resulted from the blocking of sight line by the trailer (12). Similarly, when the driver of the combination vehicle (1) makes a right turning in driving to actuate the lighting of the right direction light 8 (15) provided on the trailer (12), the processing unit (3) connecting with the right direction light (15) simultaneously receives the actuation signal coming from the right direction light (15), and then the processing unit (3) drives the right camera lens (24) to shoot the right side zone of the trailer (12), and the images shot by the right camera lens (24) are sent back to the processing unit (3). Next, the processing unit (3) sends the images of the right side zone of the trailer (12) through a conductive wire to the display unit (4). In this manner, when the driver makes a right turning, he can see clearly the right side zone of the trailer (12) through the images shown on the display unit (4), so as to avoid the danger of sideswiping with the other car or running over foreign matter or pedestrian due to formation of sight dead angle resulted from the blocking of sight line by the trailer (12). In addition, when the driver of the combination vehicle (1) stampedes the brake due to traffic condition or traffic signal control to ignite the tail light (16) of the trailer (12), or when the driver moves the shift lever to R gear to reverse the combination vehicle (1) to ignite the tail light (16), the processing unit (3) connecting with the tail light (16) simultaneously receives the actuation signal coming from the tail light (16), and then the processing unit (3) drives the rear camera lens (22) to shoot the rear zone of the trailer (12) and the images shot by the rear camera lens (22) are sent back to the processing unit (3). Next, the processing unit (3) sends the images of the rear zone of the trailer (12) 9 through a conductive wire to the display unit (4), so that the driver can see clearly the rear zone of the trailer (12), so as to avoid the danger of colliding with the other car or running over foreign matter during braking or reversing. Further, when the driver of the combination vehicle (1) completes turning, braking or reversing and turns off the left direction light (14), the right direction light (15), or the tail light (16), the processing unit (3) which is electrically connected with the left, right direction lights (14,15) and the tail light (16) simultaneously receives the turn-off signal, and then the processing unit (3) further drives the front camera lens (21) to shoot images of the coupler (13) and the images are shown on the display unit (4) for the driver to grasp the status of the coupler (13). In this manner, even the coupler (13) is loosened to cause separation between the tractor truck (11) and the trailer (12), the driver can grasp the status and thus take necessary disposal, thus possible traffic accident can be avoided accordingly. Furthermore, when the driver wants to monitor road status on the left side, the right side or the rear side of the trailer (12) without actuating the left direction light (14), the right direction light (15), or the tail light (16), the manual control unit (5) can be used to issue the actuation signal to actuate the left direction light (14), the right direction light (15), or the tail light (16), and the actuation signal is transmitted through a conductive wire to the processing unit (3), and then the 10 processing unit (3) drives the left camera lens (23), the right camera lens (24) or the rear camera lens (22) according to the signal to perform corresponding actuation, so as to take images of the left side, the right side or the rear side of the trailer (12), and the images are transmitted to the processing unit (3) and in turn sent through a conductive wire to the display unit (4). The driver can switch to, according to his driving needs, to see the images of the left side, the right side or the rear side of the trailer (12), so as to makes the vehicle driving safer. While the present invention has been described in detail by a preferred embodiment in conjunction with accompanying drawings, it is to be understood that above embodiment and drawings are not to limit the aspect of the panorama monitoring device for trailer of combination vehicle of the present invention. The trailer (12) of the combination vehicle (1) can be a trailer for camping or a trailer for cargo loading. Furthermore, according to program planning of the processing unit (3), the images shot by the front camera lens (21), the rear camera lens (22), the left camera lens (23) and right camera lens (24) can be shown simultaneously on the display unit (4). Equivalent variations and modifications made by the person skilled in the art without departing from the spirit and scope of the present invention should be considered to be still within the scope of the present invention.