Well, I'm not certain if the C-class seats have the same or a similar setup to that found on my 124s, but my front seats have smallish (maybe 4" x 5") plastic covers for the metalwork on either side of the seat. I found that when shifting my weight in the seat (as in turning a corner), both sides would creak, particularly the seat with the seatbelt latch, as the seatbelt was pulling the plastic cover that much more. I just inserted some felt material (discreetly) between the cover and the seat material, and it has worked thus far. You can find this stuff at any craft store, or you may come up with a more novel material (maybe a rag in a trial effort?)...