just remember that ground loop isolators and direct-to-battery grounding are not always the only options. you can also try grounding to different points in the car, or, if everything is grounded to multiple points, connect everything to a single point. or connect a length of wire from the outer portion of the rca cable to the ground cable.
unwanted noise in a sound system is usually the result of what is called a difference in ground potential-- which basically means that some devices have a easier path back to the battery then other devices. the entire idea is to get everything onto the same ground potential, which usually will correct the issue.
there are tons of different solutions to ground loops and unwanted noise, so if you happen to have a particularly annoying one, i suggest searching sites like realmofexcursion.com, or diymobileaudio.com, or the12volt.com. all those sites specifically cater to car audio systems, and will have tons of info on ground loops and the previous solutions that people have used to correct them.