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Although fan speed is controlled by a resistor in series with the fan motor, those fans draw a serious amount of power. You would need to find the ratings of the fan speed switch in your car to find out what values they used. A 10W to 25W potentiometer can be purchased after finding out what values produce minimum speed for your fan. You could get several 10W 100 Ohm and 10 Ohm resistors and experiment. No resistance will be maximum speed, obviously. To use a much lower power resistor you will need to use a transistor with a heatsink or an encapsulated fan speed controller module. Like this one available on Amazon: ELEGIANTFurmores3317 This is a 90W PWM motor speed controller that will run from 12V and cost about $7 US.
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