US code requires the neutral to be grounded at the building entrance, and the ground, well that should be grounded, shouldn't it?
For vehicle electrical "diagnostics", start by making sure you have a known good reference - use chassis ground or battery negative and you can't go wrong - the other thing is to have an idea as to what voltage you should be looking for at a particular location, and that is where most folks seem to get things screwed.
In this case I would use chassis ground as reference, and start with the tail lights only on, and probe the wiring at the socket - there should be +12V on only one of the three wires - anything else indicates a miswire or an open ground.