Help for install of factory '98 sub/amp into '91 system w/ door amp

[Mr.Mike]

I recently purchased a '98 model factory amp and subwoofer for $20.00. I have never installed a subwoofer, but the price was right. Can anyone help me with the installation? My current system is a '91 with an amp in the rear for the doors. It does not have the factory stereo,if that matters. It has a pioneer CD player which I did not install, and it has Kenwood KFC-HQ570 speakers which I did install, however I must have done something wrong because when I adjust the fader fully to the rear, they all four fade out. Oh well,maybe the prior owner screwed it up and not me. any help wiring the new amp/sub would be greatly appreciated.