The problem that I have is this: 1991 SL 300-24 with Air Conditioning (euro).
The heater will work only once the thumbwheels are clicked onto full setting.
If you turn the thumbwheel controls back off maximum it only blows cold air. Also the blower is fine on settings 1,2,3,4,and 5 but when put onto defrost (max) the red light comes on but cold air only blows.
The duo-valve is fine, this has been checked,and cleaned. Am I missing a component or sensor somewhere that may be faulty.
On full--the heater is red hot, off full it is cold, nothing in between.
I would appreciate your help.

Regards,
Ray Waker
U.K