The aux fan's low speed circuit doesn't respond to ambient or system temperature, it responds to refrigerant system pressure. The FSM suggests the high pressure switch triggers the aux fan at 20bar/290psi. Bridge the pigtails on the high pressure switch with the engine running and compressor engaged. If the aux fan doesn't come on, it could be a bad relay, bad step-down resistor, bad fan or bad wiring. If the aux fan comes on, the relay is fine but it tells you nothing about the switch. I suppose you could put a gauge on the high side but I don't know how you force 20bar at the receiver/drier without the risk of overheating the engine.

Sixto
87 300D