hi
Are you meaning the blower fan for the heater and aircon etc? Or is there a separate fan in the U13 dash for something??
If it is the heater / aircon blower fan, there could be something in the fan blocking it and the fan motor only has enough power to overcome whatever is holding it back when on high power.
On the U12, the fan is easily removed - take the glove box out ideally, though lying on your back with head in the passenger footwell you can see the fan motor. There are three screws holding it in. It is easily removed.
I had mine out recently, a few leaves in the fan but otherwise OK. Gave the motor a quick clean and lube (airgun and WD40) and put it back in.
Have a go at removing it for a clean and see how you get on. Sorry, cant be much more help!