There are a few posts in the archives on how to fix this. It is a broken toothed belt that can be fixed with a strong cable tie.
Do a search under headest and you should find all you need
OOPs, sorry I was thinking of the front headrests, still you might find an answer there with the modifier rear
92 500e, 85k, silver/gray,all stock.
96 Club Wagon,Hypertech chipped( College Hauler)
97 320 SL 50k
1980 BMW e21 323 Baur conv.
1990 300GE 99k