That someone was me!
It sounds like you don't have the factory amp... if you said there was not a silver box under the passenger's seat, then you don't have it (obviously). You can just use the mitsubishi harness (Metra harness # 70-7001) and plug it in. I have had a couple of cars with the "Infinity sound system" but they didn't have an amp. I think it just has another set of speakers in the car, but I didn't pay enough attention to that part.
If you take out your radio, and in the main harness there are only 4 or 5 wires, you have to bypass the amp. Let me know if you want a pin diagram of the DIN cable.

"Anyone who has never made a mistake has never tried anything new."
-Albert Einstein