blower resistor is bad
If the blower works on the defrost mode only, you could have a disconnected vacuum supply line coming from the engine. If the blower motor doesn't work on any mode or speed you could have a blown fuse, burned out blower motor speed resistor, bad switch or a bad blower motor.
The blower resistor controls the speeds of the blower motor Hi-resistance lower motor speed Lo-resistance higher motor speed
Probably your blower motor speed resister is burned out.
If the blower motor does not run at any speed, and you are positive the fuse is good, then it is either a defective blower motor, blower motor speed control, bad Blower Motor Resistor Pack, or loose, corroded, or disconnected wiring. I would replace the fuse even if it looks good before going any farther. Apply 12 volts directly to the motor to see if it is good.
If the one speed it works on is high speed, it is the blower motor speed resisitor that is bad.
The fuse. The blower motor speed resistor. The blower motor relay. The blower motor switch. Check for power at the blower motor connection.
Replace the blower motor speed resister.
usually mounted on or next to blower motor
With no speeds at all, check the fuse, then check the blower speed switch and it's electrical plug.
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.
The blower motor speed resistor is burned out. The resistor is usually found close to the blower motor. Auto parts stores carry that part and it is easy to replace.
The blower motor speed resister is burned out. This part is available from your local auto parts store.