My car doesn't have a frame per say, rather it's a unibody type structure. They use a lot of epoxy resins in unibody structures which can present grounding issues. Even big sheet metal panels, section of the body these days are thinner and may not actually be all that big. RIbs and other parts that support the unibody may also be glued together. Knowing this and needing substantial grounding, I run both the (+) power and the (-) ground from the engine compartment to the back and use distribution blocks for power and ground in the hatchback area of the car where all the gear is. In my set up, I have (4) 4guage grounds and (2) 1/0 gauge grounds that are connected through several distro blocks and back to the front on the 2/0 gauge wire. This configuration ensures that there is a low resistance path for effective grounding on the run back to the front. I’ve just made it a habit to do my installs this way, it’s effective, it’s clean, but it requires a little bit more plumbing, blocks, fuses and breakers and cost more for sure. As for under the hood, not sure what you are referring to. I did a 1/0gauge upgrade to the entire charging system under the hood, adding it to existing wire; I did not add another, extra ground, no need. Again, as I and others have suggested, as long as all your wiring meets or exceeds the need, you will be fine.