Not sure what all is going on with your climate control, but one thing to think about. A lot of cars are set up with full hot defrost as the "default" position for all of the actuators. The idea being, if your system goes kaput, hot defrost (and a clear windshield) would be the most important situation to have. What that means, is that if your vacuum goes away (like REAL low idle), your system goes into this failsafe mode. Same thing can happen if you blow a fuse for the climate control.
My experience with the pushbutton unit has been that the solder joints go bad. I pulled the unit apart on my 123 chasis and resoldered all the connections. Pain getting the thing apart and back together without breaking something, but it did fix my problems.
Not quite sure what to make of the solder joints in MBs. I KNOW they have (had) people there who could solder. My 1956 Telefunken Opus Royale still works and plays beautifully! Then again, they aren't the only ones. GM had a recall on windshield wiper motors for bad joints at the connectors