Alright the last vehicle I owned was a 95 tahoe. I had all kinds of speakers and subs. I haven't been brave enough to take off the radio from the 07 civic si 4d. It is the one w/ nav. Is there a way to hook up a sub? and what's the deal with the 8" (what crutchfield said, it looks like a 6 x 9) speaker in the back?

 

Yes you can connect a sub to your factory stereo. As has been discussed literally hundreds of times, you'll need a line out converter.