Defrost/vent adjustments

[Zilverado]

So I'm starting to have trouble with the truck not blowing hard out of the defrost vents at the base of the windshield - mostly blows from the a/c vents.

This was a fairly major concern the other day when I drove it back from the country in the dark in a pounding rain. The high humidity fogged to windshield up something fierce. Finally after playing with the controls a bit, the air volume seemed to increase.

So I've had the air box off before to replace the heater core, and I think I remember there being vaccuum and cable controls. I think I just need to adjust the damper, but is this a tricky job? Or would my vaccuum actuator be weak?

Any thoughts?