Ford AC/heat fan only works in 4th (high) position

2001 Ford Mustang convertible, 102000 miles. The left dial which sets the fan / blower for AC or heat from low to high has 4 settings. When it is in 1, 2, or 3 the fan / blower remains entirely off, regardless of the settings of the other 2 dials (one for temperature and the other for window defrost / floor heating, etc.). When this fan dial is set to the 4th, or highest position, it works and everything else also works properly. I've done more work on houses than cars, but I've got a good amount of tools, and I'd rather try to fix this myself than spending too much money on a car that is 6 years old. Not sure if that will be possible, but any insights or suggestions are most appreciated. Thanks, Fran