Seat belt arms
CAN be repaired by an aluminum gear made by a supplier in CA. They fit all the 124 coupes as well as the large SECs.
My passenger side also chattered and when I removed the side panel, found the gear (plastic) to be stripped. That was when I started my search for replacement parts and found the aluminum ones which are exact fits and better than the MB junk. The gear is about $57 vs. a new complete unit for near $350!