Vintage Cars: 1983 w107 vent and hot air, side vent, heater valve

Question
Just got my 1983 380sl, everything is great apart from the passenger side vent will not stop blowing hot air also the footwell gets very hot after a while.

Not got a manual yet, but could you tell how I might diagnose/fix this problem?

Answer
sound like your heater valve is stuck open . Your car has the dreaded heater valve set up,known as the ACCII ,  the big square box on the firewall with a lot of tubes and hoses going into it.  They are prone to act up . It is a system not easily fixed.  Google "Mercedes ACCII" or look here
http://www.peachparts.com/shopforum/showthread.php?t=205757&highlight=accii as a start
Good Luck