The motor will run direct off the 12v supply on position 4.
If the resistors are burning out it suggests the motor is drawing too much current.
This could be because the motor bearings are dry & binding, there could be something binding the fan ( some leaves or debris ), or the motor winding could have overheated & the windings have partially shorted.
If the latter it will need a new motor. The others can be remedied with some DIY.
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