Before you start taking the gauge out, which entails taking the instrument cluster out, which is quite a task, you could just try eliminating the sender as the problem, by bridging the earth pin on the plug to the other two pins, ground to the blue black wire should activate the gauge, and ground to blue green wire should illuminate the low fuel light, do these tests with the ignition on
If both of those work, then the problem would seem to be with the sender, which could be damaged or could just need a clean, there's tips on how to pull the sender out on one of the earlier threads. I'm not sure what condition your fuel and tank is in if it's been standing for years, i guess the tank may need emptying and flushing, if you search, again you'll get some advice
If linking across the pins still doesn't work, then you'll need to look at the actual wiring between the sender and the gauge, which goes via a 12 way connector under the dash, there's a picture of the pin arrangement in the tech manual, button above left, and then use the search function when you're on the tech manual home page. You can visually inspect the connections at both ends, as the wiring can deteriorate over time, particularly where they are soldered onto the connector pins
You could also make sure fuse 5 is ok, i think that also serves the reversing lights and brake lights, so perhaps check if both of those are still working. Sometimes the fuses need twisting in their holders, as they can build up resistance over time
