Replacing the switch isn't too bad, but more likely the blower resistor is bad. If like the earlier trucks, it sits up behind the blower motor and is held in place by a phillips screw, with a plug and about 3-4 wires going to it. Looks like a bunch of little coils.

Dealer or junkyard part.

Usually only works on high speed when the resistor packs it in as the high fan speed is direct power without any resistance.

88 V6 4Runner, 3.4 swap, SAS, front E locker, rear EZ locker, dual cases 2.28/4.7, 63" Chev rears, rears up front etc.