What is the part called which makes the ac in your car run on different speed settings(1low,2,3,4highest speed?

I just recently changed the blower motor in my 2004 Chevy Cavalier and the ac/heat only runs on the high speed. I need to know the name of the part that will let it run on the first 3 settings so I can install it. I think it's called the ac resistor, but I'm not too sure. Thanks!!!