Diagnose fan selector switch?

[Gary]

When I switch from floor to defrost to panel, etc on the heater control, it just stays blowing out of the dash vents. Kind of a pain because in the winter time the windshield won't defrost till the cab is so warm it's stifling.

I've did a search but haven't found much so how can I diagnose where and why the selector is failing to change my air from one to another setting?

If there's already a post or faq about this feel free to link it to me.