Please let me know the location of stock amp and woofer?offsets Car Audio Manifesto and FAQ - NASIOC
All the info you should need!
P.S. I use the stock amp/woofer with my aftermarket unit (Kenwood KDC-MP538U) in my 2000 2.5 and it works just fine. It DID require a lot of configuration t get it so that everything sounded good but everything works great it is very easy.