Speaker Noise

[wpoll]

The lighting system in your car uses a ~1khz pulse width modulated signals to power the lights (including the reverse lights, brake lights etc.). These signals will likely be generating noise that is getting into your stereo.

Chances are one of your audio components is on a power supply not intended for audio loads - check your power connections and make sure they are from a similar location (fuse etc.) to the factory stereo.