I can't remember exactly what the component is or what system it's tied to, but there is a push-pull type control under the lower part of the captain's control panel. It does something with either cockpit ventilation or windscreen defogging,
You're pretty close. It is to redirect cockpit air to the windshield defrosting vents. However, I never could tell it did a dang thing except to make more noise.
Now, you haven't forgot the old little u and the big U have you?