Well I finally figured this one out so I wanted to post because I hate it when people ask for help then disappear. I ended up unplugging the pressure switch and my wife drove it to work all winter, then when summer came we had an Infiniti FX so she drove that and the 300 just sat in the driveway all summer. Well this weekend I was determined to start working on the old and neglected gal. I spent Sunday afternoon removing and disassembling the pushbutton unit. There was no sign of burnt or cracked solder joints, everything looked great. I pulled all the boards and cleaned them with electronics cleaner and compressed air, as well as the switches. Put it all back and thought all was well until I drove it, the problem was still there.
Went in the house and pored over the schematic for a while then went to bed. Lying in bed I kept going over the schematic in my head when it suddenly hit me that the only way the compressor can run is when ground is supplied to it, since it is always hot at the Klima. The pushbutton unit normally supplies the ground to one side of the pressure switch, and with the pressure switch closed, the ground is complete and the compressor runs. This is when I realized IF THE WIRE BETWEEN THE PUSHBUTTON UNIT AND THE COMPRESSOR GETS GROUNDED BY ANY MEANS, THE COMPRESSOR WILL RUN.
Went out there today and almost immediately found the wire pinched at the compressor and making intermittant contact.
A butt connector and some shrink tubing later and all is well:
The AC compressor was changed by a shop some years back and I suspect the harness did not get put back properly. Is there a clip for this harness?
Anyway maybe this will help someone someday and remind everyone to CHECK THE SIMPLE STUFF FIRST.