OK, here's a different approach to the same problem.
A couple of months ago the AC started blowing hot air all of a sudden. I traced the problem to the climate control unit so I found a used CCU on german eBay and bought it. The seller said it was OK but when I installed it in my car the exact same symptoms continued - hot air only. Now I'm waiting for another used CCU from eBay and my concern is this: is something in the car frying my CCU as soon as I put it in or was it just my bad luck that I had bought a used CCU with the exact same malfunction as my original one had? should I take the risk and install the third CCU and hope it will work or should I fix some electrical gremlin first so it won't get burned? what should I look for?

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1987 300E 185k miles