Use a flashlight and look between the vents from the inside. Use a penetrating spray (one with a tube) to shoot the area.
Here is a picture of the assembly removed. It hinges at the rivets. Yours may or may not have the springs. Spray and let it sit. You don't want to use undo force because the bracket it bolts to is spot welded to the cowl inside, if it breaks it will be bad news.

http://good-times.webshots.com/photo/2261760620102454676QYmpkn