What's this for?
 

i have 2 of these (red arrow) in my rear bumper and i would like to remove the rubber flap from it cause they flap like hell when my subwoofers are playin. , should i? what is its purpose???

http://i493.photobucket.com/albums/r...l-g/bubcar.jpg

thats an air relief to allow the trunk to close properly.

what would happen if i completely seat it closed??

My guess is you'd get pressure resistance when you tried to close your trunk. You will have closed off the relief area that allows air to flow out as it is compressed by closing the trunk.

Not positive about this, but it does make sense.

especially if you have a ported subwoofer, you will created added stress on other components, possibly even creating a water leak.

sealed woofer, probably not a big deal. but it was put there for a reason. keep it the way it is.

would it matter if i always have my seats cracked open?

I believe they also help with pressure relief when an airbag deploys since there is a sudden increase in cabin pressure.

dynamat

^+1
it may be more labor and expensive, but its def. better than messing with other things in your car.