I assume your compressor
is not coming on longston. My troubleshooting procedure went like this on my project 300D. (This procedure applies to the vehicles with the Klima relay).
1) Make sure the system is charged. This can keep the compressor from coming on. Get it charged if it is low.
2) Check voltage on both sides of the pressure switch at the dryer. If you do not have 12 volts on either side then you will need to get a little deeper into the control circuit. After installing a new compressor I could make mine come on by supplying 12 v power (through a toggle switch) on one side of the EVT switch. This provided power through the freon pressure switch circuit which pulls in the Klima relay suppling power to the compressor. This led me to replace the pushbutton unit (purchased from Fastlane.com). Seems on the 300D the power source for the pressure switch is influenced by this unit. Also, underneath to the right is a relay with a fuse. There was no problem with it.
hopes this helps some with your troubleshooting