I had a mod done on my '08 Ford Nav unit. The website
http://www.nav-video-interface.com/index.php
These guys put a 2 input video connection to your radio 1 for reverse camera and the other for any other video source.
They charged me $ 450.00 for this. There is a toggle switch for the video source between the auxiliary input or stock nav info. The reverse cam input
is automatic when vehicle is put into reverse. The audio is Inline FM modulator but sounds really good. These guys turn this around in 24 hours!
Just for info if you don't want to replace your unit.