Thanks, I was looking at the backside of the cluster. The ground is on the printed circuit on the underside. I was going to jump a wire from the round connector ground wire to one or more of the mounting screws that hold it all together.
Now that I better understand how the ground works on the three gauge pieces, I don't think a better ground is going to fix my fuel gauge since the water temp and oil pressure gauges work fine.

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1987 300D 146K Original #14 Head
One eye on the road, the other on the temp. gauge

Gone But Not Forgotten
1983 380 SEL Long gone
1983 300SD Wish I hadn't traded it in on a 90' Corvette
1989 300SE My all time favorite
1995 E420 Went like a bullet