Most likely if it only works on the lowest speed then the blower motor resistor is faulty.
Here is an image that shows where it is located in your vehicle. Click on image to enlarge it for better viewing
To gain access to it :
Disconnect and isolate the negative battery cable.
Remove the passenger side cowl trim panel.
Position the carpet to access the front upper screw that secures the air inlet housing.
Remove the re-circulation door actuator.
Disconnect the blower motor wire lead connector from the blower motor resistor.
Here is a link to just one supplier of the part, there are others just search online.