I would bring it to a good car audio shop. There are so many variables with car audio (I used to be into it big time) that it is hard to diagnose a grounding problem without looking at the wiring.
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Get a ground loop eliminator and put it between the RCA end and where it goes into the amp. Problem solve. Now go get the gain set by someone with a Mead setup tool.. Save you time money and you'll learn what I learned 35 years ago, cars are hard and expensive to get right. BUT I manage NOW to keep it under a grand every time, because of GREAT low cost cool running class ds..
The Mead auto audio stuff is the bomb.. Must have for serious thumpers...