Wow, that'* a great way to start troubleshooting. When I had initially set up the audio system, I put a heavy duty ground for every single item straight into the chassis. I'll check up on the RCA cables, but I think it'* just noise from the head unit. I'll get some sort of filter as you mentioned, and I'll see if that works. While the car is off, though, nothing is heard but awesomeness. It'* phenomenal. I actually get crystal-clear audio with just the battery powering everything. When anything else power the battery - be it the alternator or another battery - then I have noise.
94 STEi and 99 SLO