Well my dream system has always consisted of:
Getting rid of the deck completely
-installing the biggest lcd i can find into that area.
-installing two more lcd's on the back of the headrest each with two a/v inputs.
-Splitting main xbox video signal to all three screens and amp for all speakers.
- Then hooking up 2 more xbox's to each of the back lcd's and the audio would go to a headphone jack on the lcd or on the door console.
- Routing the ir receiver for the main xbox to the front somewhere.
- dont matter where all three xbox's are as long as u route the ir receiver and the controllers.
SO:
-When you start the car all screens come on all xbox's come on.
-All screens are on Video1 (main xbox) which you can watch movies/music on using the ir remote. listening through the main speakers.
-You press the input button on the screens on the headrests, they start displaying the other two xbox's. with headphones on, people in the back can listen to what ever music or movies they like using the contoller from under the seat of course.
-all the xbox's will be system linked for system link games of course.
when you think what you are getting for the price of 3moded xbox's and 3 lcd's its not a bad idea.
Even with one LCD in the front and one Xbox you will have a pretty good setup.
Who needs a deck?
But you are ruling out radio/gps using XBMC