Selector for air mode stuck on defrost

Probably the mode door actuator is bad , this actuator controls where the air comes out , vent , defrost , floor etc...... May have video on youtube . MODE DOOR ACTUATOR REPLACEMENT 1999 GMC Denali

Mode selector is stuck

There is a mode selector in the plenum that controls where the air comes out. That flap inside is either broken, loose or has a bad control.

Check vacuum lines and diaphragms.

Mode door is stuck or broken .

It is all electronic these days an electric motor is controled by the selector on the dash. Remove the dash surround: gear selector in 1st engine off, tile steering wheel down, push down FIRMLY on the top of surround and pull towards you and off it comes. Remove the two screws that hold the heater/air cond. control in, there are 4 plastic clips as backup release them and gently remove the control, inspect for loose connectors (there are 2) it helps to run the blower to see if you have found the problem- I did.

This is a common problem on this site.
In your case I would look for a vac leak or a sticky blend door.
Since the direction of air flow changes when you are accelerating, I'm going to guess the blend door or doors are controlled by engine vacuum. There may be a vac leak between the dash and the engine, or the blend door may not be able to move back and forth correctly.
Defrost is the default position when a problem occurs, to make sure the windshield is clear of fog or frost.

sounds like your blower motor went out

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

You may need to replace the mode selector switch.