If I am reading your post correctly it sounds like the upper bushing for the belt damper shock has failed, which is what usualy fails. You need to replace the shock you can't just get the bushing:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AF0QVB462AF0QVEP2&year=1987&make=MB&model=300-SDL-001&category=All&part=Acc.+Belt+Tension+Damper

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