Did we forget about the cabin filter???? If you've never changed it could be what's blowing through your vents. I've seen them where there was literally nothing left to them. Over time they just start to fall apart. On the outside sealing edge of the filter there is a black foam gasket(if it's original)which could be what you're getting hit with. Also, when the filter is rotted away, ther eis nothing keeping outside particules from getting in to the HVAC system, including mice(yes, they can get through the screen). The cabin filter is easy to change and costs about $16.00 from Advance Auto. I find it unlikly that the actual sealing material used in the HVAC system would come apart like that. I've had these whole systems apart and there really isn't that much used at all. One other easy thing you can do is remove the blower motor and see if there is any thing in there. The blower motor is right under the passenger side of the dash and simply twists out.
