I have spent an hour searching through archives, trying not to start another Climate Control thread, but my guilt for wasting time at work now exceeds my interest in answering my own question: My symptoms are as follows:

Currently, my heat will turn on with the fan on HIGH for any of the selected modes on the horizontal pad, and HIGH on either high or low on the fan speeds, vertical buttons. If I press the automatic fan control button in the center of the vertical controls, it will immediately blow the #8 fuse.

Here are the details: I have a 1984 300D. The original ACC burned out a strip of the lower-most circuit board, I think because the aux. water pum shorted it out. I re-soldered the strip and put it in. It behaved exactly as it does now, so I assumed the ACC was just dead. I got a spare from Ebay, which again behaves exactly the same. Now I'm thinking something else shorted too in the great extended fuse-strip fiasco, and something other than my ACC acted as a fuse for the aux water pump.

I'm wondering if the most likely thing would be the fan speed regulator? Any opinions?

Thanks for your help

Ben