I'm gonna say blower motor resistor (usually located on the passanger side plenum underneath the dash - should have a few wires running to it) It is square with one bolt holding it is usually tan in color. When on high, the blower motor resistor is bypassed so that's why your fan still works. It's normal for the resistors to go bad since air is being blown over them all the time (cooled by the blower motor) and if there's moisture in your distribution plenum it gets on the resistor.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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