What Bartamos said about them all being tied to the same wire that's connected to the output terminal on the dimming knob on the headlight switch. Mine don't work either, and my original switch had an issue with the dimming rheostat that made that terminal dead. I happened to have a 12V headlight switch of the same style and figured I could use it by swapping the 6V circuit breaker over to it. But that didn't work out as the rheostat on the 12V switch had higher resistance, so the dash bulbs didn't work no matter what position the dimmer was in. Turns out that headlight switch has some other issues as well, so my plan is to replace the whole thing.
My bet is that your dimming rheostat has an issue causing the instrument lighting terminal to be dead. I'd pull the switch out and bench test it. You can confirm the wires to the bulbs is OK by connecting them to the headlight output terminal. They should come on at full brightness whenever the headlights are on assuming the wiring is OK.
