Exactly what Johnson said. And when he says bare that really means bare as in scrape off any paint. If that doesn't work try other grounding points, grounding everything at the same point, check your battery and make sure its all and well, if you have a signal processor lift the ground.
Do you have twisted pair RCA's? "Shielded" RCA's really don't do much. When the signal cable (or speaker crossover) is next to the power you are getting "inducted noise"