thank you for the question hope this helps
I assume you mean that the blower fan only operates on high speed, and that it also behaves the same way when you don't have the AC unit called for, just the ventilation/heater position, correct?
If that is the case, then this is a very common problem with a clear solution. There is a heater/AC blower motor "resistor block" that has failed. It is a voltage divider made up of resistors which is located so the resistors are in the airflow pathway from the outside to the inside called the intake plenum (so as to dissipate the resistive heat safely).
it is located
Passenger front side, underneath the glove box. You have to remove a plastic cover. Once you do that you are going to see a round plastic housing. Screwed into the bottom of that is the blower motor resistor. I believe it's got a 15 pin connector plugged into it. 2 small screws hold the connector into the housing. I would recommend removing the front passenger seat to prevent your back from REALLY hurting.
good luck
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Thanks
David