Here is my thoughts. Ditch the Factory deck keep it in a box somewhere, b/c by todays standards everything on that deck is a total turd. Then I would install a fold out screen deck, Most come with at least the one option of video in that is activated by going into reverse. Now how you would get both cameras to populate both images on screen at the same time would be your biggest issue.

I have found at least 20 differnet decks that do at least the one camera for easily under $250. and again even the cheap chinese decks are light years ahead of the factory system. Once I get my funds up I am planning to at least go w/ the Kenwood Marine deck b/c it has Bluetooth, matches the car, and can play most Mp3 media. But if I had my druthers I would get a folder w/ all those features, then add backup camera and some sort of Phone interface.