Another common failure is the resistor behind the driver side headlight. The resistor drops the voltage to create low fan speed. Have seen it queried more than a few times on the forum. As well, my low mileage/dealer maintained/Starmarked/owned by MB dealer management 124 came with a burned out resistor and no low speed fan operation. On this 1993 model the teltale clue was coolant temp rising to 105C, fans noisily engage, and coolant temp drops to ~95ish. Surprisingly, the A/C worked fine.

After replacement of the resistor and upgrading the wiring the low speed fans work great. The car sits on ~90C coolant temp and blows cold air all day long, no matter the outside temperature.

Since I know you're going to ask about the wiring upgrade. On the early 124 models a 2.5mm diameter wire supplied power to the resistor. For unknown reasons MB dropped this to 1.5mm on later models. The smaller wire tends to overheat and break near the resistor. Per a tip on this forum from Donnie, I replaced the entire wire from the low speed relay to the resistor and all has been well for several years.

- JimY