I have the positive going to the side post of the battery, about 4 inches from the battery I use a 2 position AGU Fuse Block with a 80 Amp Fuse, then to the Amp, all 2 gauge.
The negative drops right down through a plug in the floor of the cab, and is bolted to the frame below the cab.
I cleaned the frame down to bright metal, put a copper lug on the end of the cable, bolted it down, painted the whole thing with Liquid electrical tape.
I use a dual terminal battery, top posts for the truck electrical system, side posts for the Amp
The side post Negative drops straight down, and bolts to the frame just as the amp does.
I feel the frame is a better place to put the neg than any body panel.
73
Jeff