[Shiny]

That's a good guess at the cause for that discrepancy. Now I get why you are questioning the harness integrity.

Is there any way for you to access both ends of the harness and verify?

I suspect access is a bear, but is there any way to try and directly verify a short to ground before you dive into replacing all the harnesses?

Your meter is reading the difference so it should also be capable of reading the short.

What's the chance of disconnecting at the sending unit? If the sending unit is disconnected and the fuel gauge wiggles at all, this would confirm a short somewhere.

Does power go through the PCB before reaching the fuel gauge? If you find a short, you still need to decide if it's in the board or in a wire/harness...