When then heater or air conditioning blower fan only works when in the
high position, the most likely cause is the blower motor resistor. The
one in my 2002 Ford Mustang failed when the car was only about 3 years
old. Probably moisture caused it to fail, the part was very rusty when
i removed it. I found one online for only about $11 at
http://www.parts.com/partlocator/ind...?siteid=213787 Or you can
just go to your local ford dealer and get one. Don't let them convince
you to see the car or tell you something else is wrong or that
electrical parts cannot be returned. Buy the part and replace it, It's
a 3 minute job!

This resistor is located on the inside of the car just below the glove
box attached directly to the blower housing. If this link is still
valid here this diagram is accurate for most fords;
http://autorepair.about.com/library/...s/bl384lib.htm --------
Just look under the glove box and you will see it. It is screwed into
the housing by two screws; one connector is plugged into it. Don't be
fooled by the term resistor, this part does not look like a resistor.
The visible side has a male connector on it and when it is removed the
inside has whats looks like a small thermistor and 3 different sized
springs.

Ron Kavchok