The kids at Circuit City and other such places are the butchers of wiring harnesses.
Had the same problem on my 300D, and had to completely rip out everything they did and/or a previous owner did. Instead of using the car's fader switch, I ran new wires under the carpet and dash to all four speakers. So, I use the fader in the radio. This also allows the amplifier power to go directly to the speakers.
The moral of the story is, don't use the car's fader with aftermarket radios.