if a pen or pencil fell down the defrost, this could be it... and you'll have to dismantle the whole thing to reach it...
Or you can do this:
remove the center counsel once that is out of the way remove the push/pull plastic guard and you should see a floor vent, move the screws holding the floor vent, pull out the floor vent and you should see a motor on your right passenger area with a white circle, its held on by 3 screws remove the screws and replace