Hi
I used two camers. Didnt want extra wiring run all the way to front of the car for another switch.
I setup the reversing camera (NTSC) to focus on the tow ball/rear bumber and the other camera (PAL) on my trailer/caravan. I had to hack the TV module to allow the TV to be on whilst moving.
Use the aux unput via the TV module and power from radio module. Just do a search on the forum I worked it out from the information available.
Basically the TV camera is always powered on when the car is on. So switch to TV and go to AUX. It works well because I can change the view to ZOOM etc.
The only problem is you then have no radio/cd playing. Only used to check on trailer not to run full time.