My AMG E55 was obviously an older model but it was simply a multiple button press to enable it, you had to do a sequence of presses.

I expect yours will be a can bus signal off the speed / movement sensor or a park switch?...sounds like one for an AMG merc forum.

My Rangie has a prismatic screen so the wife can watch a DVD from her seat and I can look at the sat nav without each other seeing what the other sees. Clever stuff!