It’s anyone’s guess as to the RCAs. If any portion is in shielded it can allow in noise. Un-mount the amp and insulate the chassis from ground. Check the noise. Then run a direct ground to the battery and see if the noise changes.
Is this the only item you have at this ground point?
I troubleshot a ground loop issue years ago that ended up being a set of cheap 90 degree fittings at the back of the deck. We installed the head unit, and then the amp separately. To save money the guy we were installing for pulled the deck and ran
a set of Stinger RCA. Took 2 days to track down the cheap conectore