A/C fan motor won't blow in 2 positions?

'03 F150. The fan motor suddenly stopped blowing in the Low and Medium Low position. It blows in the Medium High and High position. What could be the problem and how would I go about diagnosing and fixing it? I don't think it's the switch in the cab, since it was operating on Medium Low when it stopped working. I turned the knob to the next higher position and it started blowing.