instrument lights out on 94 E320

[pjbakewell]

On my E320 wagon, 1994, the instrument lights come on only sometimes when I turn the head and/or parking lights on. If they come on, they stay on. But mostly they don't come on. The lights on the center panel (a/c, gear shift position, etc) do come on every time, and they change brightness if the rheostat knob is turned. So the rheostat is presumably OK. What would cause this intermittent failure of the instrument lighting, but not affect the lighting of the center panel? I haven't pulled the instrument cluster yet, because I don't know what to look for.

Guidance much appreciated.

Peter Bakewell