I just ran across this.... This could be the answer to your question about a camera interface for the factory Nav Unit. It's in the new for 2009 Peripheral Catalog on their web site.
A.Z.
Product Name: Peripheral Electronics PVCI Interfaces
Model Number: PVCI-NIS1, PVCI-HON2, PVCI-GM3 and PVCI-GM3A
Peripheral Electronics - the industry leader in OEM integration technologies and innovation – is proud to announce the release of a new line of interface modules that allow factory-installed navigation radios in select GM, Nissan, Honda and Infinity vehicles to also act as back-up camera displays. The modules (the PVCI-NIS1, PVCI-HON2, PVCI-GM3 and PVCI-GM3A) were engineered to automatically convert the color display on a factory-installed navigation radio system into a video back up camera monitor when the vehicle is placed into reverse. The installation kit for each consists of a “brain box” that features two composite RCA video inputs and facilitates intelligent switching between normal radio/navigation operation, an auxiliary video source and a back up camera, as well as a plug-in harness that allows for quick, easy conversions. Because no cutting, splicing or soldering is necessary for installation, reverting back to the vehicle’s original configuration is simple.
For specific application information, visit
Peripheral Electronics.