I have been looking at your project with great interest. It is spectacular!!
I have a 2005 Touring...bought it used w/o navigation

and am trying to do almost the same thing. I've got three little kids and we do a ton of long distance driving. So I'd like to do the mp3 and GPS up front while having videos (now and games later) in the back.
I'd like to keep the OEM head unit (or single DIN), because of wife's preference for radio, and lack of preference for the computer! I am curious if you have any thoughts on bringing GPS audio out if the carputer audio is connected to the head unit's aux input?
Do you have any updates? (Photos if you have them!!!)
Again, a wonderful project!