Wanna reactivate this thread because winter is upon us. We had the big snowstorm last Saturday and I had this issue in my Suburban. The windshield was so foggy I was worried for a minute there. After I got a diagram telling me how the blend door looked and where it was I took to it.

Well the blend door motor in this car is just ONE piece with TWO arms. When you press the button on the control you have to look at the arms as the move. In this problem the 2nd arm moves with the 1st but the door attached to the lever doesnt move. I have seen corrosion on the door handle with was tough to the touch when I went to move it by hand. I wasnt sure it was supposed to move because it was stuck so hard. But the lower one did move so I knew the upper one had to move.

I didnt have any lube spray on me so I had to use my little bottle of Marvel Tool Oil(mixed with ATF IV) and spread some on both of the door ***** and worked both of the blend doors by hand until they felt free. And then I worked the control in all of its functions watching the levers and the door ***** move. I was satisifed with the range of motion in the defrost door based on the climate control panel. Now I await the rain, and we will have a real world test.

I think this will fix all 1988-maybe 99? C/K pickups and fullbody Chevy/GMC wagons if you get whistling dash when you place it in defrost.