What you bought is for the headunit. I assume as of now you are just putting in subwoofers, which i did first also on my pathfinder for about a year till i did a full system. Im also assuming that you are keeping the stock headunit, so what you need to do is buy one of these
http://www.amazon.com/Scosche-SLC4-Spea ... B000BFL52Q
then go in your trunk and the panel on the left, take that apart, in order to take it apart quickly, take off the piece of plastic when you open your trunk and look down it runs along the whole bottom, it has fake plastic screws on it, once that is out, go over to the drivers side rear door, and pry up the kickpanel with a flathead (use a cloth under it or something so you dont scratch the paint), or if your good you can do it with your hands but dont break it. Then pull back the rubber weatherstripping that runs along the big left trunk panel, and slowly start prying make sure you dont bend it too much and your getting the clips to pop out, after that you will now be able to access the factory amp for the rear speakers (front ones have small ones built into the door). Then you need the colors for the rear speaker negative and positive wires for both ( someone on here should be able to tell you which ones they are, i forgot ), splice into them with something called Posi-Wire taps, they are a couple of bucks. Then hook up the splices to the line output converter (the box i linked you to above). Once that is wired up you will then hook the rca cables for the amp into that line output converter, run your power wires from the battery for the amp, and then ground it somewhere in the trunk under a panel or something, put in your sub, run wires from the amp to that, and you are done.
My full install thread should help you in terms to use the pictures i have on there as a reference and to see how everything comes apart.
Make sure to run everything nice and make it a professional job, dont rig it or do it half assed! With what i told you and the pictures i think you wont have a problem though. If you need any help feel free to ask and welcome to the forum.
Heres the link to my full install thread:
full-sound-system-install-lots-of-photos-t518024.html
P.S - dont let those pictures scare you that this is a tough job cause 3 quarters of my interior is in peices LOL. You only need to take off some kickpanels and one of the side seatbelt things in order to run power wire and such under the carpet.