were are you mounting the amp that you want to use the radio ground cable? You should just be able to ground it anywhere on the chaise. My amp is in the trunk and I grounded to were the upper part of the rear seats mount
My alpine alpine is located under the passenger seat. I figure that is the best spot for my ground wire. where the radio ground wire is. My amp for my subs are in my trunk and the ground wire for that is located abbove where the rear seats mount.
Ya its called a ground loop. I'd ground to the ebrake like ^^ said. Grounding to the radio will work but you want the shortest lenth possible. so the ebrake it a good spot for amps the are mounts in the cab of you car.