On my 1982 300SD(126 chassis) the relay for the AC clutch is in the fuse box. They are called "ice cube" relays because they look like ice cubes. The one for the AC is located behind four of the "ice cubes" but in a lower position. I believe it also contols the sunroof(if your car has one.)
A good way to check for a bad relay is to take a relay you know is good and swap with the ones on the car.
I have whacked each of the relays with a screw driver handle. Have everything in the on position. This will sometime unstick a stuck relay.
You should carry a spare relay, just like you carry spare fuses. They cost about $15 each.
Bill Lewallen Lexington, Ky. The Bluegrass State; where the grass is really green, and we use the green coolant.