(This seems like a good place to stick a little note restating something that I've seen mentioned before. As Hugo says the aux input should be grounded if the device connected to it isn't grounded to the car body somewhere else, like a portable CD player. If the device is battery powered that make sense, but what's a good way to proced if the aux input device is getting power from the cigarette lighter? I suppose it depends on the internal construction of the player.)

--Nathan