Actually mine is exactly the same, but I was wanting to change that to a finer control so the motor can ramp up or slow down, plus during the summer right between two settings is about perfect here... go figure.I know on mine that there are 4 fan speeds to choose from, chosen via a resistor placed on the power line to the motor. I'll just use 4 relays.
The resistor limits the current to the motor it makes it spin faster or slower. In my setup it is a single power line that goes to the motor, it is not digitally controlled or anything fancy like that so it should be rather easy to change it.
... maybe there is a chip / part / board that can do this, google will tell, but first let me finish this paper thats due in about 2 hours.