I grounded the amp the right behind the 2nd row left side seat. I believe it is attached to the chasis, and yes I grinded/sanded it down to the metal so there was not paint.
it could be that u are using some cheap quality rca ? , swap them out and run it away from the power lines, use http://www.knukonceptz.com/index.cfm
they make pretty good stuff, i used there wires in my system, no noise.