Best spot to ground an amp
 

Anyone know where the best spot to ground my amp would be? Its bolted to the back seat, so i was thinking about running ground wire down to where the metal part of the body is between the seats and hatch. Is this ok, or should I connect the ground wire to a seat bolt or some other bolt thats already grounded?

I usually use a large existing bolt, because it's going to go into a solid chunk of steel vs. thin steel in a lot of other places.