If your Heater fan will only work at one speed.
There is a good chance that it is the fan resistor, mounted into the side of the heater box
And is easily accessible from under the glove box.
Note: As A/C was an aftermarket option there are some models that have a different resistor,
so have a look behind your glove box before ordering and make sure it looks the same.
Reference: 2715006J01, ZPN-34745