I went through this sometime back. I was told by a stereo shop that the factory amp was located behind one of the rear panels. I have a 98 by the way. After removing all the rear panels, I never found it. I knew that I would have to bypass it to run an amp, so I did things the hard way. I located the factory speaker wires under the door sills, and ran new wire from there to the amp. It was only after all that, while installing the radio I found the factory amp. Directly behind and below the stock radio. Look there first.
Pull the black cover off that is located below the dash and on top of the wheel well. Look up under the dash behind the radio. If it is located there, it would be easy to see.
Now mine did not have a stock subwoofer. It very well could be different if your has a stock subwoofer.