What can I use for a signal wire? I tried using the wires that go to the rear speakers and didnt work. I'm using a rockford 400s amp it has a plug that i can use signal from wires if i splice them. I get sound and everything when I hooked them up to the headunit AND to the rear speakers, but its all crackly I'm guessing thats because its being amplified. Anyone can help me that would be great thanks. BTW Im not really a newb when it comes to shit like this Im just stumped

You kind of answered your own question. You need to bypass the stock amp that comes with the factory infinity stereo system. The amp itself should be under the passenger seat. Im stuck at work so I don't have a diagram in front of me or I would help you with what wires you need to tap and their color. I'll try to look it up later tonight if it would help you any.

You might also need to use a line output converter for a better signal to the amp. http://www.davidnavone.com has some good ones from what I hear.

The infinity system is really confusing. I acutally sat there for about an hour or so and tried to figure it out. All I really came up with is The round cable is the Remote, power, ground, and signal wire. And I Believe the wires from the big ass plug that come out of the amp are the wires that go to the speakers. I'm not exactly sure though. Any help is welcome.