How do I get my 1986 dodge ram heater fans to work?

I recently bought a 1986 dodge ram from a private own that rebuilds old cars and trucks. It's in pretty good condition. When I test drove it the heater worked fine. A month later I finally bought the truck and when I tried the heaters the fans didn't work. I can get the defrost to work a little but not very good.|||There's a resister pack on the back of the switch, which determines the fan speed - sounds like it's gone south. Can get a new one at most car parts stores....