That main loom you see branches off to the front along the inner fender, all the rear harness connections continue down the firewall and then under the cab. Nothing else is required as the harness branches all come through that large hole in the firewall with the exception of the leads to the starter you see going off to the PS (2-wires, red/blue).
https://www.flickr.com/photos/advance-design_parts_co/8129206782/in/set-72157631867396207