Try this, in the trunk by the latch, you will see a connector with a brown wire. Disconnect the connector and turn on your ignition. The gauge should go way past full. If it does then gauge is good. If not it may be a bad gauge, or bad grounds. Bad grounds can cause you to pull your hair out.
With disconnected connector in the trunk, use an ohmmeter and check the resistance of the brown wire coming from the tank sending unit, to a good ground. Hopefully, it will measure between 0-90 ohms. If not, either the black wire screwed to ground at the tank is broke or not making a good ground, and or the sending unit is bad.
Also, check this company out. http://www.fix-a-gauge.com/
This is a good cheap product to have in any Garage.