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There's a company to which you can send your Nav and they'll modify it to let the screen display an external video signal. Then you can wire up a switch to let you flip back and forth between the Nav display and your external video signal. Most of the people having this done are mounting a DVD player in the car and want to use the built in screen to show movies, but a few are using it for a backup camera display.
I don't recall the name of the company, but if you do a forum search (including the archives) you should turn up some posts by guys who've had it done. Realize that the Nav screen isn't very high resolution. It wasn't intended to display video.
They will add a pigtail to the wiring harness which will allow you to add a video in from an external source. This can be a camera, dvd player, tuner, etc.