450 SL heater problem..help please.

You don't specify the year of this car. The '76 - '80 models have an "amplifier" (logic board) which controls the system. The board's solder joints crack with age and its logic becomes insanity.

These amplifiers can be bought remanufactured for about $100. You need to find this cigarette pack size unit with a black perforated metal cover and lots of wires plugged into its base - suggest you remove the glove box and look behind it - that's its location in the 116 and 123 sedans.