The resistor in my W124/300D is behind the left headlamp, next to the A/C filter/dryer.

To add detail/accuracy to my first post, the fans turn on (high) with engine temperature (105C in the diesel), or low (through the resistor) with freon pressure. Easy way to test the resistor is to apply +12v to each terminal of the resistor directly from the battery: the rear one should run the fan on low, the front one on high. If neither work, it is likely a bad fan, if only the high-speed works it is a bad resistor.

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Gone to the dark side

- Jeff