My heater/airconditioning fan only runs on high and doesn't work on any other setting. Why does it work on high and not on any other level?

Could be a blown fuse, faulty switch, faulty blower resistor, or faulty high blower relay. Disconnect the connector at the blower resistor. With the key in the RUN position, check voltage on each terminal of the blower resistor connector while moving the fan switch to each different position. Post your voltage readings for each wire color.