RTH - call the MB 800 number and order the book titled "Electrical Troubleshooting Manual for the 300D/CD/TD". It will help you with everything you need for electrical/vacuum. OK now for some more help.
You need to find the pressure switch at the dryer and see if it has 12 volts on it. If it does not then you will need to trace that circuit out and try to figure out why it doesn't have power on it. When I got real close to resolving my problem I ended up putting a hot 12 v supply to my evaporator temp switch because the pushbutton unit was not supplying this power. I would turn my compresson on/off by flipping this switch. This supplied power (which was not coming from the pushbutton unit like it should) through the protection circuit to the Klima which would allow the compressor to come on.
I may be wrong but I thought your car had a Klima relay. Look on the inside fenderwell on the driver side in front of the fuse box/brake booster area. It should be a black plastic cover a little larger than a pack of cigarettes. Remove this cover and you should see your Klima.

__________________
Jim