When the blower motor only works on high it's usually a bad resistor.
Just like it's name, the resistor restricts power at different increments for the various speeds. On high there is no resistance. So, when none of the other levels work it's because the resistor is bad.
Hope this helps.
〉 Answered on Dec 8th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines