Think of it in this way... The supply voltage is going to be present on the gauge on one side, the circuit passes through the gauge and out the other terminal. From there it follows the wire (under the carpet or wherever) back to the trunk area and down to the plus side of the sending unit. It then passes through the variable resistance in the unit out to the other terminal which goes to ground, ie. the trunk floor.
It's one continuous line from the supply to the trunk floor. Any opening in that line will kill the circuit. It could be the gauge, the sending unit, the wire itself or any connection points between the components. All you have to do is choose the most likely suspects and eliminate them until you find the problem. Start with the easiest to get to.
I've never had to trace the wire. The only reason you would physically have to dig it out is if you can verify that the gauge works but that it's not getting continuity from the dash back to the tank. That would involve verifying that the gauge works when you check it at the dash but does not work when you check it from the tank area.
Sorry...to check the gauge at the dash, you'd have to get to the backside of it.
Check and double check the gauge portion of the circuit from the tank/sending unit.
Here's a kind of diagram...
Batt Voltage--->+(gauge)-<----(wire to trunk)--->+(sending unit in tank)-<--Ground
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Greg Reid
Palmetto, Georgia
 
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