The typical 2 post thermal gauge is connected to a voltage limiter, not 12 volts.
I don't recall where a 66 Dart has its inst' voltage limiter. If the fuel gauge is a three post gauge the voltage limiter is inside. The three post gauge 4 connections, 12 volts in, limited voltage out, sender, and has a slither of metal on its backside that sits against a small unpainted spot in back of the inst' housing. This is where the limiter gets its ground. In some cases loss of this ground was why the limiter and gauges stopped. Clean it up, snug it up and all work again.
