I'm going to put my pioneer head unit my 98 GS and I called to get a wiring harness and the guy said I would have to bypass the infinity amp. My first question is, is that hard to do? I don't really feel like paying $50-$100 just to get it done. I'm experienced with car stereos but I'm not an expert so I don't think it should be that hard but I could be wrong. And my second question is, where is the amp located? The guy said it was under the passenger seat and I took a quick peek down there and all I could find was a connector that I haven't really inspected yet so I don't know what it is for. I haven't really searched for it yet and was wondering if somebody could tell me where it was and what is the easiest way to access it. Any other information from anybody who has done this themselves or knows about it would be greatly appreciated. Thanks.

thats strange, i always thgouht they made an adapter so that you could retain the infinity amp, but i could be wrong. if you wanted to bypass the amp, it shouldnt be too hard. you would just need to run new speaker wire from the headunit to the speakers. you might also be able to run the new wires to the factory amp and splice them in there, someone posted a diagram of the amp cable pins and what they are recently. look for a post started by me about infinity systems and youll find it there.

That someone was me! :D
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.