I'm wondering if my RCA's are picking up some noise from a component under the glove box or something.

I've removed the glove box several times lately (making a replacement) and I can't see what there is back there that would specifically generate a stereo ground loop (but who knows?)

My setup is as follows: A 4G wire from battery + to a 4-position AGU fuse block (from Parts Express) tucked right in front of the battery on that little flat spot. An 8G wire runs from that fuse block, along driver's side of engine bay, through that one grommet, then right back to driver's side kick panel then under the driver's side door trim. I bored a hole in the styrofoam (it's a wagon) behind the rear seat to pass the positive cable to the amp; the amp's short 8G ground cable goes back through that hole to a nice bolt under the rear seat.

I pulled the RCAs, speaker return wires and blue amp trigger wire under the passenger side of the console, then under the rear carpet. I kept them as far away as possible from power cables as they cross the rear seat, but they do meet briefly when they come through that hole. I can introduce some hum by moving these around, but I don't have any now; I may still bore a separate hole near the midline of the car to pass the speaker/RCAs separately. Not sure what's under there in the sedan, but the wagon has a nice lateral storage pocket (just right for flares and warning triangle) that makes an easy place to bore through.

That one-and only grommet is getting purty crowded. A few weekends ago we added two 12G and a 14G (for Ham, rally computer, and driving light relay) to the 8G amp wire and I think that's about all she'll do. We couldn't see any other place to pull anything.