Hello from a new member!
I have a very similar problem with the Climate Control on my '84 380SL. I also had the system converted to R134 about a year and a half ago. Up until about 3 months ago, everything was fine. I then started having a very similar problem. Now, when I attempt to turn on the climate control, nothing. Sometimes, the defroster will come on, sometimes not. A few days ago, I removed the center dash panel and unplugged the two 12 pin connectors. When they are plugged back in, I can hear what sounds like relays (plural) clicking. I then tried again and the blower came on and worked at all speeds. However, the compresor does not come on, so it blows hot air. I have had problems with bad or corroded connections on several items (the car was previously owned by an older man and probably sat idle for some time). I tried it again today and the blower no longer comes on (as well as the compressor). I have removed and reinserted all of the relays that I can find with no results. The only connection I haven't checked yet is the connection to the compressor.
I have a subscription to AllDataDIY, but their information is extremely limited on any subject. My Chilton manuals are also severly lacking in information about the climate control. I have worked on cars since I was young, but this has me baffled. My feeling is that the control unit (pushbuttons) is bad, but at the price they are asking for this part, it would be foolish to replace it and just hope that fixes the problem.
Any thoughts or suggestions would be greatly appreciated. FYI...I tried jiggling the key, as already suggested, but this did nothing.
Thanks!
woodman72