Should be in front of second fire wall several radiator (small diameter) lines into and out. you only wan to replace insert
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1NP0SZ49Y1NP0SZFNO&year=1983&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

Should be a two prong connection going to the top or side. 4 small phillips head screws hold it in place.

__________________
Ron
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