I have an upgraded head unit in my X (E53), that allows me to run the dvd while in motion. Normally, it is just as simple as the unit being grounded out by one wire. That is how most units can tell that the vehicle is not in motion (It is connected to the hand brake or shift sensors).

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Bushman

"Do not meddle in the affairs of Dragons, for you are crunchy and good with ketchup"