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
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Or you canjust go to your local ford dealer and get one. Don't let them convinceyou to see the car or tell you something else is wrong or thatelectrical parts cannot be returned. Buy the part and replace it, It'sa 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;
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--------Just look under the glove box and you will see it. It is screwed intothe housing by two screws; one connector is plugged into it. Don't befooled 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 theinside has whats looks like a small thermistor and 3 different sizedsprings.
Ron Kavchok