It will be either the variable resistor or the fan itself.
Start by taking the fan out of the car (5.5mm socket is required) this will only take a couple of mins, its in the pass footwell under the glove box. It is held in by one 5.5mm bolt which you need to undo then twist it out while lifting a small plastic tab. The fan should then drop out. Then run 12v directly to it and that will let you know whether it is working or not. You sometimes even know before running 12v to it as you can just feel if they are seized when you try to spin it by hand.
If the fan is all working fine next thing to try is the variable resistor.