Double up the wire on EACH power supply. Like put 2 yellows(+) together on one and do 2 blacks (-) on the same one and just do it for each other power supply.
If you ground to the frame on like a SUV or something I've seen people spray the wire and the frame with like truck bed lining to keep that from happening.
Yep ground to the rear strut tower or do a run from your front batt neg.
Make sure to grind the paint off first if you do it to the tower.
For the post above me, unibody cars have no frame.
Wow I guess you can't go by specs any more it seems.
You need dual 4 gauge for power. IIRC isn't true 4 gauge inputs on the amp.
For the post above me would probably be your best bet.