This is all great info. Driving home tonight all instrument lights went out, radio, gear selector, etc. very similar to that originally described herein.
All fuses fine, all other systems work, all switches still function and light up when they should. Coincidentally, I had just figured out where the dimmer switch was last night, so I'm guessing adjusting that didn't agree with either the fuse behind the cluster or the light source. Going in for breaks Friday, I'll see what they discover.
'75 Triumph TR 6