I had the not-uncommon center dash vent mixer problem. My mechanic said I needed a new part. I later saw on this board that the problem is often the linkage to the stepper motor. I told him about this, he checked, and sure enough the motor was hanging loose at the end of the linkage.
My problem was more complicated, though, since the plastic housing to which it attached was broken off. He said that replacing it would be costly and time consuming since it would require a complete dash removal, etc. Instead he just glued the motor back in place with epoxy and, so far, it works fine.
I didn't actually see any of this, so I don't have any more details.
Thanks to this board for helping me get an annoying problem fixed!
'90 535i(m) 193K+ miles and going strong...