Cabin Air Temp Stuck On Hot

[Griz]

I have a 2000 Silverado. About once a year when I'm on a long road trip, my heater gets stuck wide open. I ended up riding 8 hours on Christmas eve this year in heavy rain with the passenger window cracked so that it didn't get unbearably hot in the cab.

 

Each time this happens, turning the temp control knob back and forth from cold to hot has no effect, shutting the truck off (like for a gas stop) doesn't help. It just blows hot air, even with the fan in the "off" position, the cab still gets unbearably hot. The radio faceplate and the AC/heater controls also get uncomfortably hot to the touch.

 

When I get to my destination and let it sit overnight the problem dissapears and doesn't show up again for another year or two.

 

Any clue as to what gets stuck and how to diagnose and fix it?