When hooking up an amp in the trunk where do you ground it?

I'm asking because I took out my system because I thought I was going to sell my car, but ended up not doing so. So now I'm putting it back in. THe ground I had before was towards the front of the trunk on the left side in a hole that was already there. I had quite a bit of noise, and I think that was the reason. Thanks.

 

a device with that much current, you should isolate the ground by itself.its best to drill one under the carpet of the trunk lining, RIGHT behind the ski-pass through. sandpaper it off.
its the best gnd i think... and keeping it short