This is a relatively common problem with these cars. You most likely have a failed blower motor resistor block. When this unit fails the blow motor will only operate on the highest setting. I would recommend replacing the blower motor resistor block and that should take care of your problem.
This is something that can be done at a shop or even done by yourself with minor tools. The resistor block is located directly behind the glove box. If you remove the glove box, you will see it held in by two screws.
I hope this helps! Let me know if you have any more questions!