03' F-150 FX, 5.4 std control HVAC

My HVAC air comes out of the defrost only, after ruling out all else I believe one of two things..
1. a broken line (somewhere) after entering through the firewall, not likely.
2. the control switch letting the vac bypass somehow.

I've tried pulling vac on the supply line under the hood just b-4 going through the firewall and get nothing, is it possible that the control switch is letting the vac pass through?
I haven't checked the vac at the switch yet but have at the control module under the dash (alongside the throttle linkage) that controls the bypass door and there is no vac, I also hear a clicking sound coming from under the dash in the area just left of the glovebox, almost like a static sound and definitely under the dash.
Is it possible that there is a check valve under the dash between the point of entry and the control switch causing the problem, and where might it be located?
Any help will be greatly appreciated.