I agree with sudden_impulse ... if grounding the wire at the connection point in the trunk didn't cause the gauge to read empty, the problem is likely somewhere forward of that point.
The wire connecting the gauge to the sending unit should be tan along it's entire length.
In addition to the connector near the back of the trunk, the tan fuel gauge wire passes through 3 more connectors on it's way up to the gauge.
One is located in the forward part of the trunk, up over the driver side wheel well. It's a 6-cavity connector with 5 wires (one empty cavity).
The next connector is located up under the dash. Above the driver side kick panel, near the parking brake pedal assembly if I remember correctly. It's a long rectangular 11 cavity connector with 8 wires. The tan fuel gauge wire is near the middle, right next to a couple empty cavities.
And the final connector is located right on the back of the instrument cluster.
Working from the back of the car forward, try grounding the tan wire at each of those connection points. If the gauge goes to empty, you know the problem area is somewhere between the current & previous test points.
If the gauge continues to read full (even when grounding the tan wire right at the cluster plug), the problem could be the terminal in the cluster plug not making good contact with the finger on the cluster's printed circuit, a break in the printed circuit trace, a poor connection where the fuel gauge stud terminal connects to the printed circuit, or a bad gauge.
The fuel gauge grounds through the metal instrument cluster housing. And since the housing is mounted to a plastic bezel, there is a single ground wire (separate from the cluster plug) that connects to the cluster. However, a bad ground connection at the gauge will typically cause it to read empty (opposite of what you're seeing).
