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If you reach up under the front scuttle panel, just above the wiper motor, you'll find an electrical connector. It's white in colour and square in shape, about 16 mm square in fact, has about 8 wires / connections on it.
Check the condition of the connections here.
Seperate that, on the side that runs toward the middle of the car, find the green wire with the black trace. Earth it there and see if the gauge reads FULL.
If it does, might be a break in the wire between here and the sender.
If it doesn't, take the instrument cluster off again, but leave the multi pin plug in. Find the same wire and again run an earth wire to it. Leave it there for a moment and disconnect it, then quickly flip it over and see if it's FULL or on it's way back from there.
DON'T try to leave earthed and turn the instruments over as you'll likely short something that shouldn't be.
If it still doesn't work, check you've got voltage (won't be 12 volts, but some volts) on the gauge screw terminal.
If you have, the the gauge is faulty. If not, there is still a fault on the back of the instruments.
There is an Earth in the loom to earth the Instruments (or there should be one there, it's a Black wire), however this has SFA to do with the gauge, just the panel lamps.
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