1993 Jeep Grand Cherokee problem

I am replacing the factory Infiniti stereo/CD system in my 1993 Jeep Grand Cherokee, with a Panasonic CQ-C1200U.  Purchased from Crutchfield, I installed it according to directions, and everything seems to work perfectly except the sound.  All other functions are as they should be.  After spending hours making sure that my speakers didn't have a short, and that all of the wiring was as it should be, I called Crutchfield.  They told me that I needed to bypass the original amplifier, which was located under the rear passenger seat.  I have been unable to find this amp anywhere .  Under the passenger seat, is the tire jack, and nothing on the rear driver's side seat.  To further complicate things, I decided to try to test the stereo in my 1995 Grand Cherokee (both are Laredo), and it installed and worked exactly as it was supposed to. (including sound)  Has anyone got any ideas why this might happen, where I might find the amplifier, or any other ideas where my problem might be? 

Thanks,

M Frazer