+1 Torkum. THe resistor is located below the glove box on the blower motor. It is held in with 2 screws and has a quick disconnect electrical harness supplying power to it. When you take it out you will likely find it to be rusted and have one or more of the three resistors blown. There is a lot of warm moist air flowing past it which is what eventually kills it. There is a functions check in the Haynes manual you can do with an ohms meter, but the part hould only cost $28 from Ford so it may be just as easy to throw a part at it. I would recomend buying the Ford part and not from a chain store.
1998 Ford F-150 XLT Flareside, 4.2L V-6, 2WD, automatic xmsn.