I will try to explain this step by step.
Fuel gauge was fine until I changed from an automatic to a 4 speed tranny, but at the same time I replaced the speedo cable so the cluster had to come out too.

I checked and re-checked all grounds and everything is the way it was before. I know my father in law had to disconnect "some" wires that controlled backup lights but he didn't mess with the fuel gauge.

Here is what happens. If I disconnect the ground wire that controls the cluster, the fuel gauge USED to drop to "E".
Now if I disconnect the ground wire, the fuel gauge remains at 1/2 tank even though it is full.

OK, If I turn on my headlights while the ground wire is disconnected, the fuel gauge drops to "E", but once I re-connect the ground, the gauge goes back to 1/2.

Can anybody make heads or tails of this??

Thanks!!!