Most car manufacturers use a series of resistors to make the heater blower motor run at different speeds. And when one or more of these resistors burn out, you're usually left with only one blower speed—high. The resistor module is easy and inexpensive to replace. It's then typically installed in the ductwork, near the blower—either under the dash or under the hood. (Refer to a service manual to locate yours.)
Purchase a new resistor module at the auto parts store (usually less than $25). Disconnect the electrical connector. Remove the screws, and swap out the resistor module. Reconnect the electrical connector and you're back in business.