thank you for the question hope this helps
your blower motor resistor has gone bad and will need to be replaced. the resistor works When you turn the blower speed control you’re directing electricity though different paths, all of which go to the blower motor. When you select low speed through mid speed you are really directing the electricity through a part called a resistor block. The resistor block has different resistors that control the amount of electricity that goes to the blower motor. But why only high speed? When the blower speed selected is high the resistor block is by-passed and full electrical power is directed straight to the blower motor. In most cases all that is needed is a new blower resistor block to return blower speed control.
to replace the resistor Lay on your back on the pass floor-remove the black plastic ducting that routes heat to the r/s of floor. there should be a black plug right in the middle of the heater box, there are 2 screws next to that plug, remove them and the resistor will slide out.
good luckBonuses are appreciated