I meant to post my question in the Electrics section, only just noticed I managed to post in Events and Meetings. Ooops.
The resistor switch dims the instruments from pretty dismal all the way down to pitch black, so I'll have to hazard the guess it still works. I need to take the clear front off the clocks and see if one or more bulbs is dead.
I had forgotten that the switches above the radio are also supposed to light up! The interior light switch glows a faint green, but the headlamp and wiper switches aren't lit up, so I guess a bulb or two in there are also dead.