, on a 1991 300TE 4matic, the aux fan is operated on two speeds. The low speed is run through a resistor (which is open on my car) which gets its input signal from the temp switch on the receiver/dryer. There is also a pressure switch on the recvr/dryer to cut out at low head to prevent running the system without refrigerant. The high speed fan circuit receives input signal from the temp sensor on the cylinder head. If the low speed circuit is working properly, you will not see the car go above 100C unless you're in slow traffic.
I'm still looking for anyone who can confirm if the temp switch is exposed to a/c refrigerant.