Even when switch is turned up still to dim, ground, switch or connections? What do you think
1988 D 100

Ground first. then switch. how are the other lights?

Upgrade to leds. You will be amazed at the difference, and you can brighten and dim them like standard bulbs.

The AC-heat cluster is bright, not sure if hooked to the light switch, now that i think about it that light comes on when light switch is pulled.

"some" led's dim, and others don't, but i can't tell anyone which will do what. best to contact the seller to find out.
the dimmer rheostat can become corroded over time, and not allow the light brightness to be at it's fullest potential.
sometimes, if the rheostat is slightly corroded, a few vigorous bright/dim gyrations might clean the resistance coil enough to allow full brightness.
quality led's will definitely make one happy, especially if one is like me and want maximum brightness.
also, remember to use the same color of led as the gauge cluster color is. blue for blue, green for green, etc.