Gas gauge not working

First step is determining if fault is sender or gauge or wiring between the 2. Removing the left kick panel to access the sender wire is a heck of a lot easier that going under the car. Of course you would need wiring diagrams to know which wire that is since there are several there.
Some diagnosis can be done via the sender wire in the trunk too. Pierce the wire casing with a shirt pin.
Where ever you test... If a meter shows pulse voltage, grounding the wire should generate needle travel to full. Keep this sort of test brief as possible since zero resistance is not good for the gauge.