Instrument cluster lights
 

I have a 1991 S4 and just recently when I turn on the headlights, the lighting on the instrument cluster which would normally just dim, does not work at all. The potentiometer that adjusts the light intensity works fine as I can see it change the lighting on the HVAC unit and clock. The lighting works fine when the headlights are off, nicer and bright. I tried looking over the electrical diagrams but couldn't figure it out. Any ideas? Thanks

See 89GT Pod Lights Out - Center Console On. It gets especially interesting at post #15.