first, welcome to the mp3car forums!
i had a similar factory setup(a 01 mitsu eclipse, remote factory infinity amp).
the mp3car forum is mostly focused on installing computers into mobile environments, so sometimes general audio questions are better answered on a dedicated car audio forum(there is a stickey in the car audio section with a list if you run stuck). you are more then welcome to stick around, and learn with the rest of us, just try to keep it in mind.
for setups like this, where you are not replacing the factory amp, or factory radio, but are adding subs/amps/etc. you will typically need to add a line output converter, and install it on the wires going to the speakers (common audio store practice is to use the rear speaker wires).
the reason that you need a line out converter instead of using the signal before the amp is because there is no easy way to tap into the signal before the factory amp--all manufacturers use their own custom connectors, and to my knowledge, there are no companies that offer solutions to tap into the wires before the amps-- therefore, there are no rca's that you can tap into in the factory stuff. though, i really wish it was that easy...