I had a problem with mine, as well. In my case, the circuit board attached to the rear wiper motor failed. Check this thread for the details:
http://www.xoutpost.com/x5-e53-forum/...er-broken.html

If none of the other suggestions here have helped, your best bet would be take the panel off of the hatch and check the voltage at the connector.

While you're in there, take the plastic cover off of the motor's control board and see if there is any "burnt" smell or if you can see anything obvious.

It's been a while since I took everything apart, but most of the pieces are held in place by clips. There are some screws holding the lower piece on, I think in the grip "handles" and maybe also behind the lights.

Hope this helps...