Looking at the wires in the photo, from left to right; call them 1-2-3-4.

Wire one is brown. This is the ground. Use a voltmeter to check the connection between this wire and the engine block. I have never seen one grounded in such a professional way, but then this is an older Mercedes and I have not seen everything.

Wire two appears to be connected to a terminal block.

Wire three appears to be connected to the other side of the terminal block.

It is possible that two and three are just mounted there and go to something else.

Wire four is the 12 v (+) to the A/C clutch. Find a 12 v power source, like the battery of the car, and run a line from the (+) pole of the battery to the black wire. The clutch should engage with a loud snap.

If so, your clutch is good. On to the next step of tracking back the 12 V to see why it is not engaging the clutch.

By the way.... Some of these early cars had a low pressure switch that would shut down the electrical system if all the freon was gone. You can see if you have any pressure on the system by 'borrowing' a set of gauges from an AutoZone, hooking it up, and seeing if you even have any pressure on the system.

If not then you need to re-read what Jim wrote since this just got real complicated.

There are only two subsystems to an early A/C system. Pressure and the electrical system. Both have to work, so determine first which it is and almost 50% of your work is done.