You will find that on most GM products the blower circuit consists of the switch that you physically move, a blower resistor, aka "resistor pack", and the motor. The resistor pack can fail in many various ways, one of which it sounds like you are experiencing. It can be found mounted on the duct work under/behind the dashboard.
-- GW - note incorrect email address "It's good to yell at people and tell them you're from Tennessee. That way you'll be safe." - Gary Busey
The Powers Family wrote:
On my 2000 Montana the first two speeds of my ventilation fan does not
> work...speeds 3 through 5 do work. Any suggestions where to start looking?