1995 Buick Roadmaster Heater Produces Warm Air

Heater problem
1995 Buick Roadmaster V8 Two Wheel Drive Automatic

I have tried to repair the heater where it will produce hot (regular) heat. I have been working on this problem since last winter. The (console) heater control has been replaced; new thermostat; new radiator cap; new ambient temperature sensor; etc. I have been told there is a "door" under the instrument panel area that may not be opening. I had an autotechnician put a scanner on it but could not detect a problem (code?). At one time there appeared to be a "short" because it would produce intermittent hot air then back to warm air. Any help is greatly appreciated. P.S. I resolved (I hope) my horn/security alarm problem by installing a direct horn button. Thank you.