what I would do in either of you guys case is test the reverse voltage and if that is good then check to see if there is a actual video signal or not, but I have things that accept video input to do that. if you get a black screen on anything else that accepts a composite signal then the camera ccd is probably bad. ccd is a sensor that provides the video it's in every camera out there, it's works like a human eye. or there is a break in the wire somewhere, which you could possibly determine with a ohm test.
and you could do a test on the headunit by feeding it composite video from any source like a videocamera (with 12v applied to the reverse trigger wire)