Max;

Instrumentation lighting is protected by Fuse 3, so check this first...also variable (by turning lighting knob) and this is typically intermittent after so many years...meaning that it typically works at only at the position of the variable control, where the slider control was positioned for a long time and no oxidation was able to form.

If while rotating the knob back and forth , it flashes on and off, that pretty much confirms the oxidized control condition...I suggest you simply rotate the control back and forth a bunch of times more to scrape away the oxidation...

I'm working on an article on this issue to show repair options which address this chronic issue, but variable instrument lighting can be bypassed altogether and set to maximum by tying terminals 58a and 58b of Light Switch together.

See also: http://www.volvoforums.org.uk/showthread.php?t=100943

Cheers