The aux fan comes on for two reasons. The first is a low fan speed due to the air conditioning pressure. Look at the a/c receiver dryer and locate the switch which turns the fan on. Remove the clips and jump the wires with the ignition switch on. This sends power through the resistor and should turn on the fan. If the fan turns on, then the switch on the receiver dryer is bad. If the switch does not turn on, check the resistor. I owned an 89 300e and the resistor looked good when it was really bad. Heat will break the porcelain down. I would replace the resistor just as a preventative maintenance measure.
The second reason the fan turns on is for high engine temperature via a temp switch on the engine head. This puts the fan in high speed and bypasses the resistor. I believe it should cause the fan to turn on at around 107C.