AC - Where to start looking

[tfrain]

My AC, like many in the old Mercs, is not working. The system appears to be in pretty bad condition, although the car itself is in great condition. I believe my CC electronic controller thing is working fine. I know i need to check for the foam hose to the temp sensor and do a few other things, but the main thing I need to know is this -
Where can I check using a multimeter to see if the CC system is sending a signal to the Compressor to switch on?

Is this the first thing I should check? There is nothing plugged into the compressor and I think the plug itself is broken. Would I be getting current there at the plug that goes into the compressor? I've traced the wires back to the passenger side where they are plugged into the top of the - what is it condenser (thing that looks like a can)?? - should I be getting a current there as well. Sorry, I don't know a lot about AC systems.

I've looked at the online manual, but it appears you need some sort of special mercedes diag. tool to check this stuff out.

I'm just confused as to where to start. Heat and all works fine - goes to outside temp when you turn the knob to cool.

Thanks to any experts who might want to reply!!

Tom