Probably your blower motor speed resister is burned out.
The blower resistor controls the speeds of the blower motor Hi-resistance lower motor speed Lo-resistance higher motor speed
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
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 resistor is bad
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.
Swap the relay with a known good one, many of the relays are of the same part number. Keep in mind that the blower motor relay only controls the high speed function. The blower motor speed resistor and the blower motor switch controls the lower speeds.
Should be behind the glove box. The blower motor speed resistor is located under the hood, on the firewall, passenger side, close to the blower motor.
The high speed relay for the blower motor would be found under the hood, on the passenger side of the firewall, close to the blower motor speed resistor.