Depending on the make and model it could be the switch or on some cars the circuitry includes additional components like a small circuit board. answer most likey to be the fan switch, a blower motor does not alter the temperature only blows more air to be heated thru heater
I would suspect the blower motor speed resister is shot.
If you only have high speed I would blame the blower speed resister. If you have no speeds and the fuse is good, I would blame the blower motor or the switch.
If the one speed it works on is high speed, it is the blower motor speed resisitor that is bad.
Probably a bad blower motor resistor
I would suspect the blower motor speed resistor is bad.
The blower motor speed resistor is burned out. The high speed function bypasses the resistor. The speed resistor is generally located near to the blower motor.
The blower motor fuse protects the blower motor electrical circuit. The blower motor relay operates only the high speed function of the blower motor. The blower motor speed resistor operates the lower speed functions of the blower motor.
It would have a high speed relay and it would be in the power distribution box under the hood. The blower speed resister is for the lower speeds only.
If you have no blower speeds at all, first check the fuse. If the fuse is okay I would suspect the blower speed switch and its electrical plug has melted. If the switch checks okay I would suspect the blower motor is bad. If you have high speed only, I would replace the blower motor speed resistor. If you have all lower blower speeds but no high speed, replace the high speed relay.
Blower motor control?
If the blower works on slower speeds only I would suspect the high speed blower motor relay (under the hood in the power distribution box) or the switch itself. The blower speed resistor is working if all the slower speeds work.
Only for the high speed. The lower speeds are controlled with the use of a blower motor speed resistor.