If I had to guess, I'd say its a ground problem with the instrument cluster.
The idiot lights all ground separately through the harness, but those other bulbs ground to the circuit board, which grounds through it's mounting screws to the frame of the instrument cluster, which grounds through it's mounting screws to the dash itself. Lots of places for things to lose contact over the years.
The bright indicator doesn't go through the dimmer at all.
If the lights in the heat controls/radio/ashtray/clock work with the dimmer, then you know the switch is ok.