Its likely the switch resistor (I think its called) that may have corroded over time?
It is located under the pass-side dash near the blower motor. Its held in place by 2 screws and has a connector going into it. Easy to get to. You can see it just by looking under the dash. Remove it, and take a look at the filament. It may just be corroded. I bought mine for like $15 at ford about a year ago. Now I have Low Med & High.