I am installing a new CD Receiver using a Crutchfield kit but am having trougle wiring the rear speakers. There are two adaptor plugs - one for the plug behind the receiver and the other for the plug by the steering column. Front speakers are in the doors, not the dash. When I first tried the recommended wiring I had no sound. With experimentation I found that I needed to wire the front speakers according to directions as if I had them in the dash. Still no sound in the rear, though. Should I splice them from the receiver to something coming from the old external amplifier? If so, what color wires should I be finding, and where?