Interior ac/heater vents

Have an 02 Dakota and recently regardless of where vent knob is positioned (floor/defrost, etc) air blows out of the middle defrost vent strongest followed by the middle and lower vents. The heater being on vs. AC seems to affect this. Any ideas? Thanks

Check the black vacuum line at the driver-side near the heater hoses. If it's either cracked or leaking...it will only blow out the defrost vents.