Yup, just a couple weeks ago I had to troubleshoot my son's truck. He'd struggled with power cutting in and out, but he never fixed it, so it finally lost all power. I replaced the wire that runs down to the engine and up the firewall with one that just runs over the fender. Personally, I think GM's routing is absolutely horrible and was done just to make the harness easy to install at the factory.
Don't forget that the first few inches of the wire are fusible link. It's cheap and readily available at your local auto parts st
I'd have said it was on the supply side, not the ground side. I'd be looking at the other end of the fusible link in your photo. Bad ground wouldn't kill voltage at the distribution block.
I bet the fusible link is damaged--intermittent. Thinking the one you have circled is the one that leads back to the starter motor, and from there to Battery +. But you'll find out for sure when you start tracing that larger-gauge wire.
Thank you sorry for replying late. I forgot to mention the photo isn't my own photo was the closest I can get too to what mines look like do to me not being near the truck at the time . Just recently purchased truck not too long ago from family member and noticed once you mentioned the fusible links that some of the wires doesn't have any such as the one I circled in the picture but was reconnected with a butt splice . On one end of the butt splice wire was exposed and turning green . I recut the wire but haven't had the chance to get a new one at the moment yet so since then but I'll definitely be tracing that wire tommarrow because I did retouch the wire to the panel in the picture but the problem still stayed the same with voltage dropping and going back up.