Heating control only works on high

Hello,

It sounds like an issue with the blower motor resistor.
The resistor is located on the side of the blower motor. It controls the speed of the fan as you turn the knob for different speed.
The resistor can have faulty circuit and typically fails by only blowing in the highest setting.

The blower motor is located on the fire wall, on the passenger side.

Here is a helpful article that covers the blower motor resistor:

https://www.2carpros.com/articles/blower-fan-motor-works-on-high-speed-only

Please let me know of any questions.
Thank you.