Heater blower only working on high
 

I purchased a resistor pack and went to install it and lo and behold there wasn't one installed from the factory. The spot was ther but it wasn't even cut out. I see a round gray unit with a 4 wire plug located by the blower motor.#056700-5260 with terminal 3 and 4 show a open switch and 1 and 2 look like a resistor diagram. Does this take the place of the resistor pack? If not what else would control the speed. Any help would be appreciated. Thanks. Bob