1. I grounded the sensor connector directly to the battery, turned the key on, and the temp gauge didn't move.

2. I pulled the bulkhead connector at the firewall and verified there is continuity.

3. I checked resistance at the sensor and it's at 0.2 ohm, so the sensor is grounded.

4. I inspected the bulkhead connector and reinstalled it. With the sensor connector still grounded I turned the ignition on hoping the bulkhead connector was maybe loose previously. No dice.

It looks like my problem is inside the firewall.