look at the advertisement at the top of the SL forum, it shows an advertisement for the Unwired Tools digital servo upgrade. It sounds like your control servo for the heater has gone bad. You can replace it with a rebuilt for several hundred, or get the digital upgrade unit for about $700. The digital upgrade servo is guaranteed for 5 years, and is the way to go if you are going to keep the car. In any event you should go to the unwired tools website and download their vacuum diagrams, which is what you need to check the vacuum connections. Any vacuum leak in that system will kill the heater. The first sign of a vacuum leak is that the blower will only work in the defrost position. Your car has what they call the ACII climate control, and relies on the control unit, that unit under the hood, to change the fan speeds, control vacuum flaps, and control the water valve's on or off position. It usually only lasts a maximum of three years or so.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C