The monovalve is between the firewalls, near the center and has a small 2-wire connector. Here is a link to a photo:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1W71C3A361W90NKHVR&year=1984&make=MB&model=300-SD-002&category=R&part=ACC+Mono+Valve&appChassis=_any
The most visible part is the plate with 4 small screws. There is a rebuild kit which is about the easiest job you will ever do on this car. Remove 4 screws, replace innards, & restore.