Adding an amp in the back: place to ground, 12v on?

I'll be adding an 800w amp in the back hatch storage compartment. The amp has a fan so it shouldn't get too hot under there?

Would like to know if anyone has had success finding a good location to ground the amp back there - ideally within 18" of the amp and must be a place to screw in to bare metal. Also, any tips on preventing rust if in fact a new hole needs to be drilled? I think I read spreading silicon or something?

Also, I've been informed by someone on a car audio site that the PAC-OEM2's generation of an amp 12-v on (pursuant to it sensing audio present on any speaker wire) may or may not work.

In case it doesn't, does anyone know a switched 12v source in the back and if not, what's the consensus on the best one to use overall? Still pin 3 (power - acc on) on back of the radio (but not pin 4 which is just "power" - and not switched?)

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