hey guys. i have an 04 sliverado regular cab. this has ben hapening for a while now. never really cared until we go 6 inches of snow lastnight... my heater is blowing cold air no matter where the sliders are. it just goes back and forth between hot and cold whenever it feels like it. i have noticed that it seems to work better, not good but better, when i am driving at higher speeds for a longer amount of time. however, when im stuck in traffic because its snowing like crazy, its nothing but cold air. any help??