Heating Cooling system blows HOT constantly

Hereâs the situation. I have a 1999 Chevy Blazer with the 4.3 Vortec V-6. I have had numerous heating-cooling problems in the past. This was all related to the Dexcool anti-freeze and the fact that it doesnât work with this particular engine (this was told to me by the lead mechanic at a Chevy dealership). After numerous flushes and having the radiator boiled I finally gave up and changed the Heater Core and radiator myself and went with Prestone Green instead of the GM recommended DexäCrapä. This solved my problem for two years. Then about a month ago I had the tell tale signs of a bad Heater Core again and wasted a day pulling the dash and replacing the Core. Once I got everything back together everything worked fine except the AC. The Heating-Cooling only blows hot and it blows hot whether it is on or off. In fact it blows so hot at times while driving I have to move my foot off the gas peddle to keep it from melting. The engine temp is fine and the blazer runs fine overall except for this one little problem. As we get into summer here in the Midwest itâs going to become a huge problem. Does anyone know what might be causing this problem??? I dread the thought of having to pull the dash again to try and figure out what might be wrong.