Okay, you determined that the AC clutch works and system blows cold.

For the work-around, you could theoretically put a simple toggle switch
in the cabin and run 12 volts to the compressor through the switch.
Sort of how the 300D works with the old ACC from the factory.
Ground the other lead off the compressor.

Do that until you solve the problem.

Behind the passenger side headlight is the AC Receiver/Dryer with
pressure switch. Take the two wires off the pressure switch and jumper
across them. If the compressor engages then this pressure switch
is bad. To replace it the system has to be discharged.
So I might recommend leaving it jumpered and remember to replace
it the next time the AC system gets discharged.

There is not much else to this system. I don't remember anyone ever
needing to replace the brain that is up under the dash, but it probably
is responsible for sending the signal to the compressor.

Maybe somebody else here knows for sure - does the AC compressor have its own relay (the ice cube shaped things) ?

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250