I believe it's part of the "driver alertness pack" that all M3W's are delivered with - free of charge!
I should add that this feature saved my life in Norway. On one of the very long days in the early hours of the morning I hit a bump which woke me up (yes I'd fallen asleep). Thankfully this happened about 3 seconds before I was going to hit the armaco, on what looked like a large drop.