I have an 03 1500. Does not have split control heater. My blend doors are fully functioning, have no issue with air directing to defrosters or floor. I drilled a hole in the heater box to check door. I drained some antifreeze out and ran with cleaner added to water for two weeks. I then flushed heater core, got a bit of rust out of it, ran garden hose to both sides multiple times until nice and clear. Drained by lower radiator hose and added new antifreeze. Let truck warm up to temp and had hot air blasting through heater/defrosters. Test drive down the road and then a gurguling sound from dash, and heat goes away, barely warm on lowest blower setting. These get air locked? Cap no good? I’m back to where I was before, but I had it working on the first part of test drive, doesn’t plug up that fast