Its that black plastic thing behind the driver side rear quarter panel... It has flaps and if I blow on them I can see light....its still consealed behind the bumper though. I wouldn't be able to take it out w/o removing the bumper I don't think. At least w/o breaking it. It seems to me its a worthless POS Pic 1
Pic 2

 

I'd say that's for ventilation, so trapped air can get out, and to limit any vacuum/pressure type effects

 

Yes, it is called a pressure relief valve.

 

Is it important? I planed on building a custom fiberglass box and I wanna use all the space I can. This is in the way of a good amount of extra space I could be using!!! I wanna rip it out and cover the hole. It prolly contributes to all the road noise anyhow. The thing is covered by carpet and behind the folder...so I don't care if I get pressure back there. Its so restricted by so many things that it prolly doesn't ever work.

 

I am pretty sure it is designed to keep any fumes from staying inside the car. As the car moves forward a small amount of air is able to be purged from inside the car. IMO, I would leave it there to help guard against any possibility of issues with carbon monoxide.

 

It is also there to help shut your doors.

 

It also helps keep mildew from forming as it allows moisture an escape route.

I'd leave it intact.