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As close to the amp as possible. You can use an existing screw/bolt or put in a new one but your ground wire should be as short as possible. I put my amp under the back seat (not the best location for heat control but mine isn't very powerful anyway) and I used one of the holes in the seat support and just added a bolt through it. Don't forget to remove the paint or you won't have a connection.
2001 XJ build thread -- [URL="http://www.jeepforum.com/forum/f22/01-xj-grunge-munkee-build-1298918/"][COLOR="Red"][FONT="Showcard Gothic"][SIZE="4"]Grunge Munkee[/SIZE][/FONT][/COLOR][/URL]