These symptoms, and a little common sense as to the sorts of errors an
installer might make, suggest that the ground connection for the gauge is
not connected to the airframe ground but rather to the power source for the
internal lighting. When the internal lighting is turned off or is set to
max dim, the source voltage is very near ground potential, so the gauge
works properly. At high or intermediate brightness settings the source
voltage is closer to the power bus (battery +) voltage, so the voltage
across the gauge is insufficient to allow it to work properly.

This should be easy to check with a simple voltmeter, or even an ohmmeter

-Elliott Drucker