Letting smoke out of the wires.

Good grief ! Neither of those gauge studs is connected directly to chassis ground.
One of them is connected to a pulse voltage from the limiter.
The other is connected to the sender. The studs, muts, printed circuit board, are all parts of these current paths.
When 12 volts from the amp gauge wires went there that current had a path through the oil sender at about 80 ohms. It also had a path to ground through the closed contacts in the mechanical limiter. From there it had a path to ground through all other senders via the printed circuit board and those gauges, nuts, etc..
You're welcome to PM me if you need my help. Good luck