Did you install heat shields? They do not last very long without the heat shields (ask me how I know):

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ424F01ETLO&year=1985&make=MB&model=300-DT-001&category=All&part=Mount+Heat+Shield