This is typically caused by a bad resistor. Could be the switch itself, but it's usually the resistor.
What about it? Need more info.
Probably your blower motor speed resister is burned out.
faulty blower motor resistor
You most like have a bad blower motor resister.......
blower resistor is bad
Not knowing what year S10 you have, you can generally find the blower motor high speed relay nearby the blower motor on the firewall. The resister for the other lower speeds is also found close by the blower motor.
The blower motor relay for high speed would be located in the power distribution box under the hood.
The blower motor speed resistor is burned out. The resistor is located close to the blower motor and is easy to change. Your local auto parts store will have that part for you.
try replaceing the switch that controls the low to high for heater It's gonna be the blower motor resistor (near the blower motor).
Need to know what year. FYI, the relay for the blower motor only operates the high speed function. The lower speeds are operated by means of the blower motor speed resistor.
Blower motor resistor defective
If the one speed it works on is high speed, it is the blower motor speed resisitor that is bad.