Has the motor ran at different speeds when it has been able to work? Could be alot of dust from the brushes in the motor housing. mine was clogged up. Just totally stopped working. It's really easy to remove the glove box and access the fan motor.
Check the wiring that goes onto the motor for voltages when on fan speed 1, then 2 etc upto full speed. If that shows up ok, remove the motor (4/5 crosshead screws) and there is 3 rubber prongs that hold the motor into the housing, push these back with a flat head screwdriver whilst pulling lightly on the motor and it will come out.
The brushes are held in a clip, remove this and pull brushes out.