The irony of grounding a system to a metal frame that moves around on rubber wheels is not as ridiculous as it sounds. You are not really grounding anything on the car, you are connecting them to a reference voltage (negative battery post) so the potential will be the same at all devices.
Thank you, thank you, thank you.
I can hardly believe that I've been grounding things in cars for something like 25 years now and that simple truth escaped me all these years.
Makes total sense though, now that you say it.