I was working on the truck today and seen this in the vent by your feet.  I am thinking it is the door for the defroster?  I think this has been off since last spring because I remember hearing something rattle.  I could never figure out what it was.  I did not notice anything unusual, but I have it set to blow air out at my feet 95% of the time or I have the windows down.

Edited February 2, 201411 yr by ABennin

That clips on the top of the vent control shaft. I'm not sure of its exact function but I do know its for diverting air and in the control of it. This is not part of the blend door.

 

The tab on the bottom hooks the main door below it so make sure of your position before placing the top cover back on the HVAC case.

I don't actually have it all apart right now.  I was working under the dash and saw a shine from the light.  I was looking in from the passenger side and could see this in the vent and pulled it out with little effort from a needle nose pliers.  I was looking in the factory service manual, but it hard to tell exactly where it goes by the picture.  I will have to take another look at what you mentioned and see what exactly this one does in regards to operation.