The fuel gauge is basically a simple ammeter circuit, that is, the meter is measuring the current thru the fuel sender’s variable resistor ( floating resistive arm…).
+12vdc is supplied to one side of the gauge/meter (from the ignition switch), the other side is carried, via a pink wire, to pin 6 of the 12 pin terminal strip within the engine control pod. From there it’s connected directly to the tanks variable resistor/arm via the center or main terminal at the tank ( fwd inboard part of tank). From there, current flows thru that var. resistor and is returned to -12vdc/ground. From the ground point on the tank, there should be a second wire which carries that ground to either the engine pod or main engine (don’t recall which). The engine & engine pod have a solid #10 ground wire between them.
Diagnosis is probably easiest by staring at the