Benzmeister, you most likely have a vacuum leak. The Climate Control II system in your '79 will not function with a leaky vacuum pod, the loss of vacuum will cause the vacuum operated switch that turns stuff on when a button is pushed to fail to operate. If you remove the glove box liner and start checking for leaky pods you will probably find that there are several leaky ones - my car had several, completely prevented the heater/AC from operating. The default operation with leaky pods is lots of air to the defrost, and if you push the AC on switch, it will get cold, but no heat at all.

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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