Your stock amp is located inside the driver's side rear fender. What you need really depends on what you have. We could use more details and background in order to help. Like HU / amp / speaker specs, etc.
Rooz (and others), jump in anytime here . . . got a feeling this thread's gonna be a long but good one
I think this could be a good candidate for Harry's idea about extrapolating a DIY on car audio, especially since folks always ask how to convert the rears to 6x9's . . B3M2W5i, if you have a camera, might be cool to document the whole process?