Heater works on high setting only?

Hello,

It sounds like you might have a faulty blower motor resistor.
When you turn the fan speed knob, the resistor on the blower motor changes the speed.
Over time the resistors can wear off and go bad, sometimes you might loose one or two speed settings, other times only one speed will work. A lot of times only the full blast will work.
The resistor is mounted at the bottom of the blower motor. To access it you will have to remove the trim pieces under the glove box on the passenger side.

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

I have attached the diagrams for your reference.

Please let me know of any questions.
Thank you.