2002 GMC Safari Heat/AC Problem

My 2002 GMC Safari only blows heat & AC air out the defrost and lower foot vents, stopped blowing out the main "face" vents. I've tried a new heat/AC three knob control panel and the problem still exists. Any ideas what I can try next ??

Follow your vacuum lines, inspecting every inch, including the boots. I'm willing to bet you have a broken/rotting/cracked vacuum line or two. Common on these vehicles.

If your fingers turn black when you touch them, it's time to replace them.

Your vacuum line will be discontinuous (broken) right in front of throttle body base- access it from in front after you remove the air cleaner box and air intake snorkel-- use windshield washer tubing (sold at anywhere) to splice back together any salvageable black plastic tubing- mostly it will crack in your hands where it has been exposed to heating/cooling/heating for years....
I've seen maybe a few dozen just like you describe...