hmm, where did you get the 100W spec from? to my knowledge, in 2002 they replaced the normal 6 disc in-dash head unit to the Rockford Fosgate 300W system (has subwoofer built in to rear side panel.) anyways, look into that and post later. In the meantime look at this page:
http://www.installer.com/hilo/
This is a sketchy page but it will put you on the right track. get a wiring harness and a hi low adapter (aka line out converter or LOC) to tap into the speaker outs from the head unit before it goes into the factory amp (if you are in fact running the fosgate,) otherwise just send the speaker outs into the LOC, out to the aftermarket amp, then out to the speakers.
** you might want to open up panels and actually take a good look at the factory speakers as many posts regarding the fosgate system warn that they run at varying ohmage (tweeters @ 4, dual cone 6.5s in the doors at 1ohm per cone, 6x9s at 2 ohms and subwoofer at 4 or something) basically, most people on this forum just recommend installing new speakers in conjunction with a new amplifier for fear of blowing them out.
waltman