In over the rubber fuel line that hooks the sending unit to the steel line
If it's the factory one, it's a metal strap that clamps onto the neck of the steel line coming off the sender unit and onto the steel fuel line.
If you ground the sending unit wire, the gauge should read full. If that works, the issue may be ground or the sending unit. As Moparal mentioned, floats can leak and sink to the bottom of the tank. Or you could have bad sending unit.
If the gauge doesn't read full then the guage/wiring may be at fault.