I had that same problem and found a faulty connection back at the camera. It was the Composit Video feed connection. I was able to put a scope on it and see by wiggleing it the voltage dropped to about .25 Volts which puts the sync out of spec. Mine had RCA Type connectors but yours may have a SVGA type connector. Anyway the +12 VDC was fine and the Video signal was reduced. That signal is only 1 Volt Peak to Peak into 75 Ohms so it doesn't take a lot of resistance in the connectors to reduce it into unstability. I'd check up at the dash first only because it's eaiser to get to in most cases, but the camera end is probably more exposed to the weather and will be the most likley to go bad.
1999 Tradewinds 7372 Cat 3126