My dealer said you can not buy knobs for the R129. You have to buy the entire vent. It was a while ago, but I think they quoted $80 per side and $200 for the center!!!
I painted the little metal nubs left with black paint and said the heck with it. I am still bothered by them and keep looking for someone with a doner car, but it sounds like everyone has the same problem.
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Coast Aero Support Services, LLC
'92 MB 500 SL
'01 BMW 525i Sport
'98 Harley Davidson RoadKing
'74 BMW 3.0 CSi - Gone to a good home