could this be causing an issue?

x2. From looking at the 79 wiring, and the comments from 66plyValiant, I'm sure you can troubleshoot these just like our A's

Basically what you have there is switched ignition feeding the regulator which has a ground terminal, and the output of the regulator goes to any gauge except ammeter, ie, temp, fuel, and oil if equipped.

All three gauges have the same movement electrically. This means you can use the same resistances to troubleshoot them.

A radio shack 25 ohm resistor ( 4x 100 ohm wired in parallel) after a minute or so with the key on should result in a 1/2 scale reading, substituted for the temp, or fuel sender.

If only one (fuel gauge) does not do so,.........bad gauge.

If both don't read correctly, probably bad instrument regulator, assuming wiring is OK.

Wiring and connections are also an issue.

But the needle not going down is a big clue.