Check your heater hot water control valve (duo-valve);solenoid valve which controls the flow to each side of the heating; when faulty it defaults to full flow, probably one side of yours is faulty therefore constant hot flow , to check disconnect power lead at duovalve then both sides should run hot no matter where temp.controls are set.
See here, http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1SL15BCKH1SL15C9I7&year=1998&make=MB&model=E-320-005&category=R&part=ACC+Mono+Valve.
Geo