1990 Chevrolet S-10 AC/heater blower

First check to see if it is getting power when the switch is turned on then check the relay underhood then check for a blown fuse. It may be the switch is bad.

If the fuse is blown is why the blower won't work, yo probably have ahorted wire someplace or the motor is bad if it'snot new that is why.

If it is getting power then the motor is bad was it replaced with new or off another vehicle. Or something isn't right like a bad ground wire from the blower to ground it is usually a black wire. If the ground is good and motor gets power it should work Or is it working and not going through the vents? That can be happening as well. You have vacuum actuators that open different doors so check by the defrost as that normally works if it's a problem like that you have to see if it has a a vacuum leak or a bad actuator. If the fuse is not blown it's highly unlikely that the blower is bad more like it's not getting power due to a bad relay /or resistor or switch etc.