I ventured in to replace a burnt-out bulb on the center vent dial on my W124 and wound up sitting in the kitchen staring at a completely disassembled vent housing.
It took an hour or so to get it put back together, and even with that being a learning experience, I doubt that the process is complicated. I got the dial sync'ed up with the mechanism in the dash, but checked the operation of the dash linkage before reinserting the vent housing.
I would think your unit is binding somehow, but not missing parts. Furthermore, I think the job is far less expensive than you've been led to believe, but then again, my dealer wanted $125 to replace that bulb!