Where does fuel guage get its power?

[66clow]

Trying to diagnose fuel guage. The wire to tank and at fuse box has no power to them. The connector below the fuel connector on the fuse box what is that to? It has a constant 12v on or off and it is not labeled. I ran a wire from there to the fuel connector on the box and the guage went to full. So guage seems ok I just can't locate the 12v power source to power the fuse box to the tank . If I am reading this incorrectly please let me know.

Thank you