I assume this is the 85 300D of which I know nothing. MB AC systems of this vintage typically have a refrigerant low pressure switch at the drier. It'll be the switch with contacts directly on it as opposed to the high pressure switch, if so equipped, with two pigtails. One wire at the low pressure switch comes from the PBU, the other goes to the Klima relay. Test for continuity across the pressure switch then use the wires to determine whether a signal can get from the PBU to the Klima relay. One wire should have continuity at one on the pins in the Klima socket, the other should be open across the board. The wire that's open across the board comes from the PBU. I don't know in your car whether the evaporator temp sensor is wired downstream of the PBU or one of the sensor inputs to the PBU. This should tell you whether the problem is in the cabin or under the hood.

Sixto
87 300D