For some reason, the compressor on the 81 380sl I own started running constantly whenever I push DEFROST button, even though the compressor button is OFF, and hot air is coming out the correct vents.

The compressor is also running constantly when I push the BILEVEL button, and the temperature is changing when I move the temperature wheel. So I know the wheel is working correctly.

I also know that the compressor button is working because the compressor is coming on when I push it and the AUTO HI or AUTO LOW buttons, and the center flaps are opening as they should. But in this case, pushing the compressor OFF button turns the compressor off, as it obviously should.

I've replaced the servo and the amp (twice) with units I know are good. But I've still got the problem, and I don't know where to look next. So any help that anyone could give me would be greatly appreciated.

Thanks very much in advance.