let me go look at my Bentley and trace it back a minute.
L10 cluster lights get power on T28/12 (pin 12 of the cluster plug). That goes through U1/14 to what the Bentley calls terminal h1. H1 powers lots of things including the lights in the rear widow defogger switch, so you can check power on it by observing those lights.
H1 gets power from R/4, which gets power from pin 58b of "Light Switch". This pin supplies power from the dimmer wheel, which it gets from pin 58e, which comes from fuse 3 (10 amp).
So, get a cheap/free multimeter from harbor freight (or find/buy/make a test lamp) and verify power at those locations. When you find that, say, 58e has power but 58b doesn't, you know the thing in between (i.e. the dimmer wheel) is defective.
It's usually fuses or bulbs though