Like most things on these cars, the culprit can be one of a few things ... I would say to test the push button unit (PBU) voltage readings by the brake booster on the firewall (under VIN number plate and water drainage on driver's side I guess you could say) ... with key turned to on and defroster setting, red wire to black wire should be about 12 volts, yellow to black should change from 4.5 volts to 10 volts depending on high low setting on PBU ... different readings from this and PBU might be culprit ... the other possibility is the blower motor regulator is shot .... that is located under the blower motor ... you could find a DIY on this site as it is located way under the wiper in the engine bay .... to test that, apply 12 volts directly to the blower motor and if you hear its loud as hell, then the regulator will need to be replaced .... this should help too
http://www.peachparts.com/shopforum/showthread.php?t=251372&highlight=test+push+button