2001 Suburban Stereo Wiring Problem

OK so I have a aftermaket HU to install in a 2001 suburban, I can get the factory amp to turn on but I can't get any sound from the rear speakers. The front ones work great but none of the wires from the harness seem to give me anything in the rear of the vehicle. Am I missing something? Is the a second amp maybe that I need to power on as well? Would it be easier to just bypass the factory amp or what is the best route to go to get sound from the rear speakers. I could care less if the volume control and such from the back seat works at all. Any ideas or help would be greatly appreciaited!

try to plug the factory radio back in and make sure the rear radio is not turned on, if it is, turn it off then reinstall the aftermarket.    I've never had that problem, but that is what it sounds like.  Otherwise you can bypass the amp it should be located behind the glove box.

Thanks, I was already past to the point of no return (guy I was installing for was to cheap to buy the harness) so I just spliced the wires on the amp, and all works amazing! Once again thanks for the quick reply I appreciate it!