Mercedes Benz repair: warm engine-cold car, number search results, typical cause

Question
I just bought my first Mercedes.  It is an 86 300sdl diesel #121.  It is cold here now and the engine temp standing and driving heats to 80c and the inside temp warms up too while stopped but when I start driving the vents get colder and colder until it's just cold air.  The blower is blowing and the temp. is on MAX. and the engine doesn't drop temp.  What part is faulty or what is going on.  I don't believe the thermostate is stuck open as the engine temp stays at 80c

Answer
Typical cause for this issue is a failed mono valve diaphragm.

Climate Control:
http://www.mercedesshop.com/shopforum/showthread.php?t=142408

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1986 Mercedes Benz 300SDL
Part: Mono Valve Repair Kit
 Note:  Heater Valve
Electromagnetic heater valve.     
R3023-15174 Mono Valve Repair Kit Bosch IN STOCK $55.50  
  
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=merc...