Hi from Russia,
I spent significant time fitting Zune HD in My '05 BMW 330XI (E46).
T tried to keep original BMW 16:9 board monitor (BM), use it's buttons to control Zune and show player interface on the screen. BM is fully controlled by IBUS all user inputs (button presses, volume and navi roll rotations, etc.) are broadcasting in IBUS. Together with my friend we made custom controller with IBUS interface which reading BM button presses and sending commands to Zune dock via IR. Zune HD dock was also modified: IR receiver has been unsoldered and moved to dash for remote operations. Dock has been installed in box under armrest. Now I could control Zune with both BM buttons and standard remote.
To show video on the screen I connected dock's composite out to rear camera input of BMW TV module. That input could be easily activated remotely by pulling one of the pins to the ground.
To get as much SQ as possible Zune HD dock connected to external DAC Audiotrak Dr.Dac2 DX via optical cable. DAC is connected to Bewith R-205S AMP (Japan domestic audiophile brand). Stock BMW CD and Radio (BM54 module) are connected to analogue line-in input of the same DAC. Thanks to good external DAC, Zune HD sounds much more detailed and accurate than factory CD transport (and most aftermarket CD units too).