So I've done some further research and the biggest hurdle is to find a GPS unit with video input. The following models do have the video inut, but all are old, outdated and/or discontinued and come with their own problems.
Garmin Nuvi 5000
Magellan Roadmate 1700
I don't understand why some of the newer models don't offer a video-in feature? Not even the very expensive ones! Seems I'm SOL for now!