The key is to test on the yellow/white wire going to the fuel tank sending unit, (yellow arrow). If that wire is grounded, such as rubbed through and touching the body, the gauge will read "Full" all the time. If there's a break in that wire or in the sending unit, the gauge will read "Empty" all the time.
If you look at the third drawing, they show a connector under the rear seat, then the wire goes through the floor to the tank. That connector would be a dandy place to perform the tests. Disconnect the plug, then on the part that doesn't go through the floor, use a small jumper wire to ground the terminal inside it. Be sure the other end of the jumper wire is touching a paint-free, clean spot on the body sheet metal, or the head of a clean, shiny bolt.
With the ignition switch in the "run" position, which should make the gauge read "Full". It should read "Empty" when the jumper wire is removed.
Images (Click to enlarge)
Monday, January 31st, 2022 AT 5:51 PM