ok, i have a Sony Xplod MP70 cd player, i wired a harness up and tried it..Radio Works but CD player doesnt..so i put my buddies Pioneer in, and same thing..i even tried different harnesses..why wont the CD Player work? stock is back in and it works fine...

 

It doesn't power up or it powers up and no sound? If no-powerup condition - check harness, you need at least 3 wires connected, there supposed to be 1 ground and 2 "+" wires, one is Accessory (without it connected unit will not power on) and second is fused bigger wire - actually "+".

 

If the radio has sound but you get no sound when using the CD player it sounds like you might be using the wrong blue wire to turn on the factory amps. You need to make sure you use the blue wire(usually blue/white) for amp turn on not power ant blue wire. This is a common mistake. Hope this helps.