i had this same problem on my 85 300d, check your klima relay, the 85 300d was the first year for this change, the klima relay is on the left inside fender, just in front of the fuse box, under a black plastic cover, pull it out and see if any lands are burnt off the circuit board. these things are expensive, around $200, if yours is bad, i have a extra brand new one ill sell . email me if you want it.

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matt