My amp is mounted to the back of the passenger rear seat and the ground is run to the chassis under the rear seat. The ground should be connected to the chassis, and most amp companies recommend you remove the paint between the ground and chassis. You could run the ground to the area behind the trunk panels and find a screw to connect the ground. Probably wouldn't be more than 1-2 feet away. I'm not an expert but I've heard a lot about making sure you ground to the chassis.
it will ground fine there. it is part of the chassis. how big is the amp? most companies dont recommend mounting amps hanging upside down. it causes the heat to radiate back into it. i wouldnt worry about it if it was under say 600 watts
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