I am finally in our hotel for the evening. I did not get into the wiring for the power for the trunk for the backup camera and transmitter for the wireless video signal transmission. Also, on both of our 73 Mustangs we do not have the center vertical bezel mounted gauges (we are using Dakota VHX analog gauges in our instrument panel). If you have center mount gauges our approach will not work out for you. In shrt, we have Garmin Drive 51 GPS units in our Mustangs, with the optional backup camera. Be care to get the backup camera kit with the receiver needed for the display unit. There are to different backup camera kits. One has the GPS power cable with the video radio signal reception, and costs more than the supplemental camera 2-4 kits. The Camera 2-4 add-on units have only the additional camera and additional camera transmitter.
To get power back to the trunk for the camera and video transmitter. One way was where I ran an auxiliary wire from the fused side of a 20 amp fuse on the fuse block back into our trunk. On the other Mustang I got a headlight relay from RocketMan (mini kit) and set up the headlight relay to power the headlights and parking lights on any time the car is running. Then in the trunk I merely tapped into the parking light circuit to draw the 12 volts needed for the backup camera circuit. No need to run a new wire to the trunk, plus now I have running lights on all the time.
Here is the YouTube on our GPS Drive 51 unit installed. I hope all that helps.