so i did some searching to find some answers for my own questions that i wish to share
http://www.discountcarstereo.com/detail.aspx?ID=434
that is an adapter that will work on either side of the signal bus cable going from the factory head unit to the factory amp so you can use it to use a new head unit with the existing amp/speakers/wiring or you can use the factory head unit with a new amp and any wiring you want from there
and ya the factory amp's under the passenger seat
i still dont know if this "MR400692" factory Infinity amp unit also contains the crossovers for the conponent speakers or not so if anyone knows, let me know