Hmmm, sounds like that box could be what you are looking for. In the diagram there are two wires connected to it, I'd have to look again to confirm. If you've tried all combinations, maybe that piece of the puzzle is just fried? Sorry I can't be of more help.
My suggestion now would be, that since you have all the trim pieces off, just take it over to a bmw shop and ask them what that box is for, they shouldn't charge you anything for that, since you've done all the work so far. Or again, maybe someone on another forum can be of more help?
Let me know what you find out.