http://www.zippydan.com/pictures/IMG_0230b.jpg

squared in red

i took off my rear bumper to fix a dent. saw this thingy that appears to have a filter. i let it get rained on overnight. just wondering what it is and if i should replace the filter sometimes :-)

civic 05 sedan rear drivers side

 

I would have to say it is the breather for the trunk. It allows air to flow in and out and keeps the pressure in side it equal.

But I am not 100%