Fuel gauge

68 Barracuda Guel gauge,
So usually for the last few cars,I take the wire off the sending unit back by the fuel tank ground it and the gauge goes to full. It’s usually the sending unit every time.
On this particular car, I did the same same test with no change? Gauge?
This is a picture of the back of my gauge Panel.
I noticed a condenser wire off. Does anybody know which plug it may go on and will it make a difference?

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