I greatly improved the brightness of the speedometer and tachometer instrument lighting in my '72 1800ES by paint the inside of the housings with bright white spray paint. This looses the blue tint which was created with a blue paint pattern, but the lighting is now much brighter. Of course it is incandestent white-yellow, but that is fine by me.

It sounds like you may have a separate problem with poor connections somewhere along the line. Have you checked for ground voltage drop? Put a good DVM between the case of an instrument and a solid ground when the instrument lights are on. The voltage should be nearly zero, if not then you have a resistive ground somewhere.

John