Any number of things could be wrong but you're overdue for a PBU or at least reflowing the solders in your PBU if it's original.

For AC, check for continuity across the low pressure switch at the drier behind the left headlight. Usually the switch without pigtails is the low pressure switch. If there's no continuity, you're low on refrigerant and the compressor won't engage. Could also be a bad signal from the evaporator temp sensor. I forget if it's an interrupt along the compressor wire or a trigger into the PBU. Could also be a bad signal from the cabin temp sensor but defrost mode should override.

Sixto
83 300SD