Automatic-switching reverse (back up) caravan camera
Composite video signals are low enough bandwidth that you should be able to switch them with a relay. Detecting when to switch is a different animal. However, I think you may be able to get away with a simple peak detector. The video signal should have sync pulses in it that should be relatively easy to detect. So I would recommend using a peak detector with a relatively long time constant followed by a comparator and small transistor to drive the relay. It would probably be advisable to ac couple the video signal and run it through an amplifier, possibly even a single transistor amplifier, to limit interfering with the video signal.