S-70 Some Dashboard Lights don't work

I've a US spec 2000 S70 w/2.4 Non turbo engine and Auto transmission w/
91,000 miles on it. The problem I'm having is that some of the dash
lights work and others do not. Don't work: Speedometer illum, gear
shift lever illum, rheostat indicator light, headlight switch indicator,
and clock. Even though the rheostat light itself doesn't work, the
rheostat still controls the rest of the illum lights on the dashboard
area. Work: Two fog lamp switch indicator lights, all radio indicators,
heater control indicators, glove box, and so forth.

Thought some wire connectors must have separated, so we had a mechanic
trace all wiring using Volvo S-70 wiring diagram. He removed the
dashboard, traced every circuit, found no disconnected wires. He tested
or replaced every single bulb - even those which don't turn on - and all
were good.

Does anyone have an idea about what might cause SOME but not all of the
display lights to fail? Might a defective rheostat cause this condition?