Tom, locate every wire in the rear third of the car. Make sure you look under the car, too.
Cut all the wires at an imaginary line near the rear door post. Next, follow the cut wires to the devices connected to each wire. Unbolt, unscrew, or rip out the devices and collect them in a pile.
When that is done, call Kim Hanson and ask him to come over and look at the debris pile. Kim is a ace number one stereo guy, and he'll spot the amp right away.