Use a hi-low line converter so you can get rca output to your amp. Tap into the rear speakers just make sure that the connections are correct, meaning positive to positive and negative to negative. It's not hard at all. I installed a kicker amp 1200.1 with 2-12 kicker comp R subs and the bass coming off them is pretty hard hitting! If you decide to install an amp with more than 1000 watts you might want to install a capacitor as well. If you don't expect your lights to dim at loud decibels and your car to run rough with your mpgs dropping a bit as well. Mine ran rough, I could tell the rpms would drop when playing the system loud and, now that I've installed the capacitor the rpms are steady and the car runs just like it did when I drove it off the lot. Good luck and hope u love your loud bass as much as I do!
P.s. Just like Cody said install your amp and cap on the 40 side of the 60/40 rear seat split. It's more convinient there. You can either use a 14x14 pc of particle board (as I did, covered it with black speaker box fabric) screwed into the frame of the seat then just screw the amp and cap tight to it or just screw the amp and cap direct to frame of seat. The board is easier to do, a little more work but it looks much better.