Temp gauge mystery

"Ground" is a huge mistake in older cars made back then. Engineering should NEVER have depended on lamp shells, ECU and VR cases, or any other part of a component for ground return. Every one of them should have a dedicated pigtail. You'll notice that on modern cars, very few if any, components are grounded via the component shell or body. I still remember some years of '60s Chevs with part of the tail lights in the trunk, you always got an unpredictable light show out of them.