Seeing how A8 parts routed it, I don't see how this can be done without takeing the front and rear seats out together and lifter the passenger side carpet area....
As for routing, they poked it past the passenger near-side rear seatbelt area (near the parcel shelf) and then routed it alongside the wiring loom which goes sort of past the passenger front seat if i remember.
1 thing i'd recommend is to test the audio as you can get interference in the cable (get a decent cable to help with this and route it properly) which can be heard as you rev the engine.
The effort is all well worth it though for the video connection you get. If you're just looking to plug in your MP3 player, it may be worth just getting a
dension gateway 300 that plugs in the back of your headunit, allows for digital sound quality and doesn't require all the wiring?!
-Oli