Well, the number 12 fuse contains:

Stop lamp, turn signal lamp, instrument cluster, (relay auxiliary fan term. 86, relay power wind. term. 86, cruise control tach, control unit/heat. water valve for climate control system.

The AC compressor and blower motor are located on fuse 8.

The only thing I can think of is that perhaps the "relay auxiliary fan term. 86" refers to the Auxillary AC fan. I suppose that switches on whenever the AC is running. Possibly I have a short to investigate there?

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1981 240D 4sp manual. Ivory White.