Both radiator fans run together, either in low speed mode (triggered by the a/c pressure switch) or in high speed mode (triggered by engine temperature). Have you tested the high speed fan mode? Unfortunately I can't give you exact directions; it's one of the sensors on the thermostat housing.

Carefully check all the wiring for the aux. fans, especially around the preresistor. In the early 90s MB redesigned the 124 wiring harness, and the wire feeding the preresistor dropped from 2.5mm to 1.5mm. On my 1993 300E that 1.5mm wire failed several times at the junction with the preresistor. Presumably due to heat from the current load, perhaps in combination with conducted heat from the preresistor. I replaced it with a 2.5mm wire (surprisingly easy to do!) and all has been fine since.

- JimY