Instrument Cluster Voltage Regulator

That panel doesn't have a lot of light in it to begin with. The later model bulb sockets and Sylvania #2825 bulbs will fit ( see pic ).
The bulb sockets get their ground through the screws beside them creating a path from copper trace on the board to the gauge housing. Then the housing is grounded through the screws in the dash. All of these hardware links can become loss of conatct points. Cleaning every little piece would help but.. actual copper wire is a much better conductor.