I have a 82 300SD and experienced climate control problems shortly after I purchased the car in 88.
I found that all the problems I had were caused by the poor design of the Climate Control Module. The problem was the soldered connections between the three printed circuit boards within the unit were broken. The boards are held in place by the layout of the module.
I corrected the problem by resoldering the connections, extra heavily. The connections between the boards should either be made using contacts or wires. Solder connection between boards is hobby shop quality.
I have not had any other problems with the Automatic Climate Control since. If I do, I will redo the connections by drilling holes through the copper lands and use wire.

Another electrical problem I have had and easily repaired is the aux fan in front of the radiator. The problem on this was simply frozen brushes. The fan was dead, even with 12 VDC placed across the fans power leads. I disassembled the unit and found the brushes frozen in place. That looked like they had only worn down around 25%. I broke them loose, cleaned the brush guides and have had no problems since.