The compressor will engage in any mode except OFF and EC.

Disconnect the low pressure switch (the one without pigtails) at the dryer behind the driver side headlight so the compressor doesn't engage. Removing just one wire will suffice. If the compressor engages, you have a problem with the Klima relay. If the compressor doesn't engage, set the control to defrost and feel for warm or hot air. If you get warm air at idle and tepid air at speed, your monovalve is likely at fault. IIRC the 124 monovalve doesn't have a replaceable cartridge like the 123 and 126 monovalve. I don't know if it can be rebuilt by mortals. If you still don't get heat, disconnect the power to the monovalve. This should leave the monovalve wide open for coolant flow to the heater core. Still no heat? That's a really unusual situation.

Sixto
87 300D