Your problem different than symptom covered by extended warranty, see here
FAQ - Blower works on high only , but indicates problem in same area; i.e., switch, wiring or resistor(motor not involved). Inspection of resistors and wiring thereto suggested and relatively easy (pass footwell on heater duct} inspection of switch wiring requires partial dismantling of dash/console to gain access (effort varies by make, model and year).
Blower switch on high bypasses resistors by providing a direct, but parallel path to ground for ground side of motor. If this path was open then blower works on low, only.
Perhaps simpler to explain this way: if connector is pulled off blower speed switch blower will work on low only. Alternately, normally pulling connector off of resistors will still allow blower to work on high if speed switch on high.
Running a direct ground connection to BK/R terminal of motor will cause high speed.
