Why does your kia sportage blower work only on high?

The ballast resistor is burned out. The electrical wiring provides one voltage, 12 volts. The ballast resistor has two taps, one at the far end and one half one. The one half way provides the medium speed and the one at the far end provides the slow speed, by cutting down the voltage reaching the fan motor by simple resistance. A dealer will probably be able to provide it, but below is a related link to a website that sells the blower motors and resistors.

I've replaced mine (2001 Sportage) twice before checking further for the root cause of the problem - a blower motor that was stiff and drawing excess current. The resistor is located under the right hand dash near the blower motor, above the passenger's knees. It's held in by two screws. The blower motor itself is held in by four screws. Remove both, check the blower motor for free turning and if it is stiff, replace it along with the resistor. A source for both parts is rockauto.com