2000 Silverado AC issue

[Gullettmatt]

I have a 2000 Silverado 1500 4x4 with center console. No dual climate control and manual controls. The mode door does adjust to different area of the truck as normal but blows weak. After further inspection I found out that the vent through the console that leads under both front seats for the rear blows all the time no matter what. It must blow 75% of its power through here. Also the heat and ac works fine as well. Please help can't figure this out.