You've got a bad or weak ground to the cluster.
Try getting a hold of the connector/plug-in and giving it a wiggle.
Lights are grounding through the fuel guage with key off. The ground path through the guages resistor is what's making the lights dim. Power on the dash light ground wire is why the fuel guage drops with key 'on'. When lights are off, the fuel guage is grounding back through the lights.
Clay
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