Get the Panasonic MXE CQ-C9800U if you want the most powerful head unit on the market today. It produces 60x4 RMS (ignore max ratings as they're useless). They go for around $350 on eBay.

An optional step 3 after HU replacement and front speaker replacement would be an 8" free-air sub in the rear deck powered with a small amp.

1999 Ford Contour SVT #900/2760 (SOLD) 2005 Mazda 6s 2003.5 Mazdaspeed Protege 1998 Ford Ranger 4x4