According to NEC 551-4(c) bonding voltage converter enclosures, the noncurrent-carrying metal enclosure ... shall be bonded to the frame of the vehicle with a No. 8 AWG copper conductor minimum or equivalent. It further states the grounding conductor for the battery and the metal enclosure may be the same conductor. There are many other sections re: low voltage and battery grounds but the only low voltage items that can use the vehicle chassis as a ground return path are the vehicle's exterior lights ie: stop, turn, tail, markers. Any other low voltage circuits must return to the distribution panel's neg. bus.
