no sound from rear speakers

I don't know what to do. I have a 1997 Ford Explorer. I am not getting any sound from the rear door speakers. I had an aftermarket stereo and cd changer in the Explorer. I lost sound on three speakers. I found out after the fact that you have to bypass the amp in an Explorer to use an aftermarket stereo. I reinstalled the OEM radio. I proceeded to replace the blown speakers. Puting new speakers in the rear still did not get any sound. Happened to put one of the rear speakers in the front and found out that it works fine. I now have great sound in the front, but still no sound in rear door speakers. The amp is working because it is powering the radio just fine. What do I need to do to get the sound back in the rear speakers? If I have to get to the amp to fix the problem, please send detailed instructions on where it it located and how to get to it. Any help would be be greatly appreciated.

On another note, does anyone know where I can get cd changer modulater controller to work with a Kenwood KDC-C717? I Would still like to use this cd changer with this Ford radio.

Does yor explorer have a factory subwoffer in the right rear quarter?
If so the amp is located on the subwoffer enclosureand yes the right rear quarter trim has to be removed from the inside to remove it.

there should not be a problem with the amp from hooking up an aftermarket head unit, shouldnt being key here. Amps are designed to take the input signal and amplify it, most aftermarket amps are taking in on average a signal at 50 watts rms. Anyway to test take a set of speaker wires, any two to three foot section and hook them to the speaker lines on the head unit, and hook that to a speaker, test all of your outputs from your head unit. if those all work then you know you have an issue with the amp. as for bypassing you can either trace out the lines and splice around the amp or you can direct wire under the carpet or along the trim and then into the door.