This is a VERY easy repair....
If your fan isn't working on all speeds, the fan motor resistor is most likely what's faulty.
There are two types, one if you have automatic temperature control, and one if you have manual.
This is the manual one, Motorcraft part #3F2Z18591AA.
One Ford dealer quoted $22.17, another said $36.00
1) Open the glove box beyond it's stop by pressing the right side in, till the rubber stopper clears the side.
2) This is the part to replace.
3) The screws are 8mm.
4) Slide the part out....it's credit card size.
5) Press the center tab to remove the connector from the part.
Reverse these steps with the new part, and you're good to go!