I had exactly the same poblem with my 82 300SD. I bought the repair kit which included all the plastic gears, however it looked like it was going to be a major pain to replace the broken flap so i drilled a hole thru the broken piece in to the flap and epoxied in a piece of steel wire. Has been working fine for years. I would also recommend changing all the vacuum cannister diaphragms while you have the dash apart, I didnt and was back in the dash 6 months later.

Bob H.