I think funola just did an alligator clip to lock down the monovalve and then fixed his ground-to-pin 4 trace to stop the "all-heat-all-the-time" syndrome.

The monovalve gets +12v from the fuse box and the CCU provides the ground path to close it. ID the hot pin and the ground pin and see if you can get to close in positions other than "O". If so, check your traces in the CCU or research funola's thread.

Monovalve which fuse?

It's not that simple. Depending on the mode and temperature selection, the ground is facilitated by either the CCU (via numerous paths) or the electronic unit for temperature control.

Just did the re-solder trip to mine - and burnished the relay contacts with 600 grit. Every time a button is pushed (the 5 'middle' ones), it stresses the solder joints. Put on your squinters, grab a magnifying glass, and head outside - indirect light - bet you'll see hairline cracks in the switch solder points...

but since you're in there, might as well re-float all the solder.

For the purposes of simply getting the monovalve to close and stay closed during the record setting Texas heat and drought any ground path will work - as long as he selects the correct pin from the monovalve.

Is this a permanent repair? No. That's why the second and third sentences recommended additional troubleshooting. If applying a +12v DC differential - with the correct polarity - across the monovalve doesn't get it to close, then there's a problem there and no amount of troubleshooting or repair of the other units will stop the heat.

Do you have any helpful advice for the OP to reduce the amount of hot air in his car? Or do you want to parse terms on CCU, PBU, circuit path, etc?

I'll even throw in a bonus reference for his light bulb question, but that's just me trying to be helpful: http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1416673-w123-bulbs.html (No, I didn't exactly answer his question, since he wanted LED info, but it's a step in the right direction)