If you are looking for space, the pioneer shallow mounts are pretty decent sounding subs, not too pricey either.

 

Yes, an LOC (line output converter) takes the speaker leads and reduces them to RCA levels. Some amps have the ability to take speaker leads directly for input as well. The problem is that many factory car radios automatically reduce the amount of bass when the volume is turned up to protect the 25cent speakers put in at the factory. The RE-Q has a circuit in it that corrects for this. It takes speaker leads in one end and gives you a full range RCA, a crossed over subwoofer RCA, and a remote turn on lead for the amps out the other end. It's pretty cool for the money.

Personally, I would leave the back speakers stock and fade them way down.