First thing to check is the low pressure switch on the dryer behind the left headlight. If there's no continuity across the contacts, there isn't enough refrigerant pressure. Or the switch is bad. With the engine running and ACC set to defrost, momentarily jump the wires to the switch. If the compressor engages, you're that much smarter. If not, check for 12V at one of the wires. 12V means you're getting a compressor signal from the cabin.

Sixto
83 300SE
98 E320 wagon