1994 Chevy Silverado, heater A/C controls work but panel is dark.

[bimmerbit-93]

I have replaced the heater A/C controller about 3 years ago with a new one from Chevy. It has worked but occasionally the panel did not light up. Now it never lights up. I checked the brown wire and read wire that plugs into the unit. The red wire for battery is always hot, and the brown wire is hot when the switch is on. Again, not lights in the panel.