instrument cluster question

Hum.
I don't think you have an "external" ground on the metal case.
If you fuel gauge works, that would rule out the 5V limiter you know.
If you wind up having pull the gauges , I would get the IVR4 while there.
http://rt-eng.com/rte/index.php/Main_Page
Your car has an AMP meter and not a volt gauge.
It doesn't deflect?
The temp sender could be bad for that item.