'94 Blower Motor or something different?

On my 94 Altima, I am having a problem with my blower motor or something else. The blower will only work on high, will not work on any other speed. Defrost, A/C, heat, bilevel & all still works, but regardless what setting it is on, it will not work on any speed except high. If I go to turn the speed down, it just shuts off all togather as if I turned the knob to the off selection. What could be wrong? How can I test it myself to see what the problem is?