When I wire up cars I treat all componets like they are mounted to a fiberglass body. Which means that everything gets a ground circuit added to it. I know that it sounds overkill but between fresh paint, gaskets and then the beginning of corrosion it will always have a ground. It really isn't as tough as you think. I install a ground block inside the car for everything to go back to, Under the hood I install a ground junction that every thing goes to. The rear for lights every thing goes to a terminal block that has a common ground terminal.
