I don't know if this helps, but I had a lot of problems with the heat and air with my 87 300SDL. It was always flaking out. I recently replaced the ECU unit. (the thing with all the buttons on it) Now it works great. Air and heat when it's supposed to be happening. You can order a rebuilt one from Peachparts here on the forum.

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99 ML320
94 SL600
92 SL500
95 E320 Cabriolet
87 560SEC
86 300SDL Grease Car
80 380SLC Euro

13 Fiat Abarth
02 Maserati Spyder Cambiocorsa
00 BMW Z3
90 Rolls Royce Silver Spur
80 Ferrari 308 GTSI
88 Jaguar XJS12 H&E Edition
99 Land Rover Discovery