If it is only on the A/C it could be the flap inside the heating/cooling housing behind the dash. The airflow from the cooling evaporator has two doors on one hinge that control where the air goes and might be stuck, or the vacum hoses that operate the vent doors might have come undone. You can refer to a service manual to find out about that. The last thing might be the vent hose that runs from the assembly up to the main dash vents in the center. Hope this helps.