You can buy the OEM back up camera that has the image on the mirror but this won't be cheap as you need the mirror, camera, and install. When ordered on the car it was like $500, who know how much afterwords. You can also buy an aftermarket setup. I don't think you can have the image come up on the OEM navigation screen.

 

Does the nav have a aux video in? Look into one of these; New Car Reversing Rear View Camera High Resolution - eBay (item 270410106794 end time Jun-17-09 20:32:17 PDT)
$ 0.98 Yes, less than a dollar but shipping is $17. I have one and it has a very clear image.

 

There are quite a few out there that attach on top of the OEM rearview mirror. My coworker has a PYLE brand in his Tahoe and it is fantastic. Much larger image than the form OEM. I think he paid around $130 for it.

 

Here is a backup camera system from SirenNet. Its a pretty cool system for the price. It has 3 inputs. I will also automatically change from the rear camera to a left or right camera when the turn signal is activated.

SIRENNET.COM - Brigade BV-770 Color Flat Screen System