Chevrolet Repair: heater fan control, resistor pack, plastic blower

Question
2002 chevy trailblazer. The heat/ac fan control switch will not work on the lowest two settings,what needs to be replaced?

Answer
Resistor pack, which will be located in the heater blower housing.
Actually, it is possible the switch is bad, but usually the resistors.
On some S10's, it was necessary to cut some of the plastic blower housing to access the resistors, but I hope GM corrected that screwup before they made your Trailblazer.
But a look in a service manual would be a good idea, and you can look at them in the public library, refference section.

Van