The key should be in ON but the AC setting doesn't matter, the engine doesn't even need to be running. The pressure switch should be near the condenser on the high pressure line, just short the two wires together that are attached to the switch and the fans should spin up on low speed.

It should be the fitting with two spade terminals on it.

The resistor should be in the engine compartment somewhere, as the fan only has two leads. You can directly test the fan at full speed by putting 12v on those leads.

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1993 300D 2.5L Turbo