The easiest way to accomplish this is with an aftermarket car stereo head unit with navigation built in. The Kenwood units use Garmin GPS software, so the nav portion looks just like your nuvi. Most of them have a specific input for a backup camera that will automatically switch over when you shift into reverse or switch the display to monitor while underway. They aren't cheap, but they are a nice slick package. It really beats trying to reinvent the wheel by trying to modify other things to make them work.
Unrestored 1969 25' Tradewind
Overkill Tow Vehicle of the Year Award:
1997 Chevy C3500 Crew Cab Dually 6.5L Turbo Diesel