My Sentra has no bump

Hey guys/gals, I am the {not so} proud owner of a 1991 Sentra. It's a 2 door model. I am looking to upgrade the stereo on it, because currently the stock stuff dosen't cut it anymore...especially considering the 120mile round trips to work. I was wondering what kind of setup would I go for. I am on a budget, but the cool thing is I gotta friend that can hook me up on the install, and he also builds his own boxes. So I have it easy there. Just give me some ideas, I am looking for sound quality here, really. Don't really care if it's not the loudest thing around.

I am also thinking of putting the subs in the place of the back seat, because I need the trunk space. Is that a good idea? Or will sound like crap???

Lemme know. Thanks.

well i suggest pioneer. speakers are good...and the HUs are really good.

subs and amp its a big thing. various types...but since its gonna sit in your back seat.. i say one 10"...one good 10".