Looking at the schematic in the 67 AIM in UPC12, that wire is designated 20DG for 20 gauge wire that is Dark Green. It runs from the temp. sensor, through the bulkhead panel and to the second pin from the edge of the connector that plugs into the gauge cluster. All along that path the wire remains DG. The schematic also shows a 20DG going to the ignition switch. If you have power at the gauge (should be DG) then I suspect a break along the path to the sensor.
I would measure for voltage on the bulk head connector wire (also DG) and see if it at least makes it there. If not then you know which area to start troubleshooting.
If you don't have a copy of the schematic let me know and I can make a copy and send it to you in PDF.
Mike