If your HVAC blower runs only on "high" chances are you need to replace the blower motor resistor (about $15). The blower is designed to run through a series of resistors on low and medium. The "high" setting bypasses the resistor. If the resistor burns out, only high will work. The resistor can be found by placing your head or better yet, a mirror on the passenger side floor mat and looking up behind the glove box. Use an 8mm socket
to remove the 2 sheet metal screws which secure it to the blower housing. Should be about a 10 minute job.
Below is a link to my Photo Gallery showing location of the resistor.
Hope this helps. - Mark