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I grounded my amps to the metal post behind the left inner fender carpet right near the power antenna ground. I made sure the paint was scraped away of course, drilled a hole and put a bolt and nut on there. Not a bad grounding spot for my 1/0 gauge.
Ground each amp to a different spot to insure no noise ( this is the best) but if you ground all amps at one place make sure that the larger wattage amp is on body of car first and the smaller grounds go on top of that.