I'm not sure but do our trucks some a blower motor resistor? we know thats a very common problem on later models when one or more of the variable speeds go out.
isn't there something like a resistor that by the fan switch?
edit:
just looked it up.
this is what you need. it's mounted to the back of the fan control area i believe
https://www.classicparts.com/1960-72...iABEgLjq_D_BwE