I also want the NAV screen to display backup camera video, so I went out this morning and removed the trim under both sides of the dash. I came to one conclusion: the back of the NAV unit is completely and utterly inaccessible. You can't even see it with a mirror. Other than taking the entire top of the dash off, I do not know how you would access it.
I am going the easy route- VR3 wireless license plate mounted camera with 2.5" LCD monitor. The rear reverse light wires are easily accessible at the hatchback, and the display should fit in the little triangular shaped cubby hole on the dash to the left of the driver.
Walmart_com had the backup camera and monitor for $99, but they are out of stock. I ordered one for $149 online at another site.
If anyone knows the trick to get to the nav unit, please respond. What I am doing will work, but having the NAV unit display the video would be way sexy.