No air out of front defogger

[vedermania]

I have a 2007 Endeavor with the following problem:

Air does not blow out of the front defogger to defog the windshield. You hear it blowing, but it is not coming out. I have switched the knob to change the airflow direction from the cabin to the cabin/feet to the feet only to the defogger/feet and the defogger only. Air blows out of all locations normally except when the front defogger is selected.

What / where should I be looking to fix this? What operates the airflow?

Any help would be greatly appreciated.