The prime candidate for this problem is the dash light rheostat.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EH0TL2AI1HX1EHF7O&year=1989&make=MB&model=300-TE-001&category=P&part=Dash+Light+Rheostat