i have a 1996 chevy silverado that the heater/ac fan doesnt seem to have the air flow like it use too, when it is in the floor position it blows almost normal amount but when it is in vent or defrost  the aiflow is cut in half or more it does it weather im using the ac or heat.also the air comes out the defrost and vents at the same time in ether position. in the max ac position the recirculation door opens but still not much air flow only time i get close to full air flow is when it is in floor position but it still isnt what it used to be  is there a cabin intake filter that may be plugged?

smoke