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[Xenon]

There's positive illumination and negative illumination.. My deck for example dimms when the headlights come on, so are meant to react to the dimmer dial on your car, so when you lower or higher the brightness of your interior lights it does the same.

Like Phi said, you should hook it up to your headlights or try to find something in your center console that lights up with your headlights and tap into it.