Look under the passenger seat to see if your vehicle uses an amp. By 2004 radio amps, and most computers on all brands of cars no longer turn on simply by turning on the ignition switch. The original radio generates a digital "turn-on" signal that is broadcast over the data buss. The amp turns on when it sees that signal.
I suspect there is something available through places like Crutchfield or Best Buy, but it would be easier to eliminate the amplifier. There are jumper plugs available for that, or you can cut and splice the wires.
Tuesday, November 15th, 2016 AT 6:38 PM