Rear Window Vent
 

What does it do? Where does it go to from outside to inside? Is it open above the head liner allowing noise, water, and HEAT into the cabin?
If so how can I close it up?

I can't answer your questions but I believe that you want it to be clear of debris and NOT cover it up. It is for ventilation, allowing air to circulate through the car.

It used to have a dark-colored foam in there, but it probably turned to dust some time ago now...

Here's the first 3 good hits on it from typing "roof vent" into the search feature:

http://forums.pelicanparts.com/porsche-911-technical-forum/401827-roof-vents.html
http://forums.pelicanparts.com/porsche-911-technical-forum/416373-roof-vent-foam-rubber.html
http://forums.pelicanparts.com/porsche-911-technical-forum/412964-replacing-foam-rear-window-headliner-vents.html

As stated by Wayne above, the vent allows for air circulation within the cabin. The flow of air over the roof creates a low pressure eddy by the rear windshield that actually draws air from the area of the vents and as it is shielded from direct verticle drops, the chance of water entering the vent is very small unless you actually direct a hose into the vent. As stated also, there was a low density foam piece inserted into the vent area by the factory that is likely long gone, though many have replaced it with a strip of foam from home improvement stores.

I've been told to be very caerful when washing the car around the rear vent openings. A spray of water from the hose directed at the openings goes straight into the vehicle! When I wash the car and rear window, I always spray water rearward off the roof.

My Foam had turned to dust and as I have a sunroof, access to the interior side of the vents via the interior headline sipper made life easier to remove the remains of the old foam. I inserted a length of universal tubular rubber door seal in to the interior gap, rubber tube type seal with one flat sticky edge along the base, the seal was slightly too big but compressed nicely in to place, worked fine so far.

can air go thru that door seal?

also, did you have to drop the ehadliner to access the vent from the sunroof opening?

Don't shoot water in it.

Cant answer if air goes through the door seal, but does not seem to be a problem.

Did not have to drop the h/liner, its a bit tight as the s/roof motor restricts access.