Here’s 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 doesn’t 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 Dex”Crap”. 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 it’s 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.

tk2aci,

Sounds like the plenum door cable either came off the swivel stud, or
it's out of adjustment. If I remember right, you can see the swivel
stud move back and forth as you move the temp control. You might be
able to access it by removing the glove compartment.
It's also possible the foam seal around the door itself is bad. If
that's the case, you will be in for a job.
Good Luck.