you need a new blower motor resistor. it is located under the passager dash and can can be replaced fairly easily for under $25 and an hour of time. It's a simple answer. When the blower motor resistor and or motor is on the way out the default is for the motor to blow on high. This was done so you would have defrost and heat and also know that somthing is up. Replace the boobs ASAP and also the resistor/temp nuts if you have one. + Answer 9/17/06 + The plastic knob may be cracked inside due to aging from UV exposure, so it is not actually turning the temperature adjustment shaft, just free-spinning. I bought new knobs for mine, at $23 apiece at the dealer, but later found that Hi-Lo/O'reilly (Texas area auto parts store) had similar knob in a GM grab bag of knobs for $7 bucks. You might check all 3 of your knobs for cracks. All 3 of mine were bad when I bought my car this year.