[ray_mcavoy]

Four things that can cause the fuel gauge to stay on empty include:

(1) Bad sending unit.
(2) Short to ground in the gauge to sending unit wire.
(3) Bad ground connection at the gauge.
(4) Bad gauge (open "full" coil).

It sounds like you've already ruled out #1 & #2. But just to be sure, you might want to try hooking up the gauges outside of the truck just to be sure. To do that, simply hook the gauge's power terminal (pink wire) to the positive terminal of a 12V battery or 12VDC power supply. And connect the negative side of the battery or power supply to the metal cluster case. Don't connect anything to the gauge's sending unit terminal.

If that makes the gauge swing to full (or beyond), you know the gauge is good. But if it continues to stay on empty, take the gauge out of the cluster and hook the battery or power supply negative terminal directly to the gauge's ground point. If I remember correctly, that's basically just a contact on the back of the gauge that touches the metal cluster housing for it's ground connection. And if that makes the gauge work, check for tarnish/corrosion where the gauge mounts to the cluster tin. If not, it's most likely a bad gauge.