Yeah, we are saying the same thing in different ways. Electrically, saying "earth" on DC voltage electrics in cars, trucks etc implies a common "ground" connection, nowadays always negative via a metal chassis, body etc. as you know, that's what is happening in your car. Just saying "negative" doesn't automatically tell you how it's created. That's why I was trying to be really clear. Some cars up until the 1950's had positive ground! Pommy of course π
So with my auto electrical background, knowing how problematic earth return/ground/common is on things like trailers and boats, I fully understand and embrace "earth return" wiring in an aircraft. That is, wire carries the current for both positive and negative. No device on my aircraft is powered via current transmitted through the metal of the airframe.