On Fri, 23 Oct 2015 19:33:20 -0400, wrote:

How does a ground work in a vehicle tail light without an electrode in
earth?

I'll step in here -
There is "ground" and there is "ground" In an automobile "ground" is
used as a term for "chassis return".Common procedure today is
"negative ground" where positive wires run from the battery positive
through switches to loads, and the negative connection of every device
is connected to the chassis - which is connected to the battery
negative. Some circuits are "ground switched" with the switch between
the device and the chassis. This is common in fuel injectors and (in
many cars) headlights.

westom demands an earth ground in his advise. How does a boat motor
run without a earth ground electrode? He does not see what Trader
explains to him. I'm not an electrician though.