I did this with my Chrysler 200.
What you need to do is go to your local auto zone, or something like that.. Or order one online. You'll need a Line Out Converter sometimes known as a High Low converter.
Once you have that, go to google and research your rear speakers.. Find out which wires on your left and right speaker are positive and negative.. You're going to need to splice those wires, and install your converter on that. On the converter is 6 wires, 2 pos, 2 neg, and 2 ground. You'll need to splice into each pos and neg on each speaker, and ground the 2 wires to something.. I used a bolt on my frame because it was close by.
Once you do that it has 2 outputs on for your RCA's, connect them to the converter than to your amp..
Congratulations, you just installed your amp without installing an aftermarket radio.