Well, it's keeping you entertained But on the upside you have been vigilant and taken pretty much the whole truck apart in your quest to fix it properly. Rust in a place like the air channel can manifest itself in much more serious problem like eating through the firewall or the hood hinge recess. I've used citric acid powder solution to clean heater cores (I pumped it through the heater core with a faulty pressure washer on continuous cycle and it really broke up all the crap in it) The heater blower units aren't too tricky to recondition. Once you pull one apart you're glad you did - you wouldn't want to breathe the stuff you find sitting in the casing or have it blowing around the inside of a cab.