99 Silverado AC works but not thru top vents

I had my 99 parked for a few days (driving my Chevelle as much as possible) and went to turn on AC. The AC is working but I can only get air to come out through the floor vents and the defrost vents. The drivers side defrost is pretty weak on airflow as well. Is it a vacuum line or small electric motor issue? Sorry if this has been posted previously.

Think they're electric on these new ones.

Check to see if you have cabin filters on that model. I have a 99' 2WD 1500 and a 99' 4WD 2500 and they both have them. Both trucks, the filters were plugged solid and wouldn't let the air flow. I just changed the filters which are located under the dashboard (passenger side) and the cold air blows great. 15 bucks for a pair @ pep boys. Hope this helps.