You have to take the motor out and take the cover off. There will be a few plastic gears inside. The one that is the culprit is the one that directly contacts the "metal screw", there should be a nice groove worn into it where it can't get any grip.

99 SVTC, T-Red, #652/2760-12.8.1998 Mother#@%@!* did I sound abstract? I hope it sounded more confusin than that!