Can we mix pwm and resistor to control brushed dc motor speed?
You need to disconnect the resistor pack. If the stock control circuit calls for high speed on the fan, You will short 12 V to ground while the low side MOSFET is turned on in the PWM controller. You do not want two controls connected in parallel. If you put the two in series, you will only be able to make the motor slower than the stock control is calling for. It may be cumbersome to try to use two control methods in series.