ac/heater low air output

All,
I have a 1992 Sl2. When I turn the ac on the vents get cold but there is a low air output. Same occurs when heat is turned on, low air output. If I turn the system from internal circulation to external there is no noticable difference. You can hear a sound change as if the vent inside did move. I pulled the blower motor down and turned the system on and there was alot of air movement.
Does anyone know what can be restricting the air flow in the system.

Thanks
997fitty

check all duct work, remove the cowl(where the wiper blades are) and clean the inlet.