1997 gmc sonoma Air only comes to the windshield sand floor. No air comes thru dash vents.
might be the control box~ https://www.google.com/#q=1997+GMC+Sonoma+heater+control+box
might be a diverter valve~ shop vac all the dust and dirt with the narrow tool so you don't get sand in your eyes~
You have a vacuum leak from under the hood inside to the heater box. Look for a black vacuum line that runs inside the firewall to heater box from the engine. You will find a crack in it somewhere.
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