How do I install an aftermarket stereo in a 1990 plymouth laser cd

player/cassette combo? Hi. i have a 1990 Plymouth Laser with a factory amp, cd player, and cassette player. I bought a two piece wire harness and wired it to my new stereo. there is a left-over wire harness, and a black round 8 pin plug that both come out from the factory amp. when i turn on my new stereo, the power just flashes on/off. i hooked up the factory stereo without the extra wire harness/8 pin plug and no sound at all. the lights come on on both decks but no sound whatsoever. so, is there an adaptor i can buy or is there an easy way to bypass the amp? thank you.
The car is in near mint condition and this is the first time the stock radio got pulled. All of the plugs are there and in like new condition. the factory amplifier is just below the bottom deck (it is a double din stereo) when i plug everything back into the factory unit all speakers work and sound great. i have an idea of what to do, which is trace every speaker wire and wire them directly to the new stereo's harness. i have hooked up many stereos in the past and have never encountered this. i was just looking for a faster/easier way to do it