LCD Backlight on Temp Control Not Working

[Slim Jim]

I have a 1999 SL500 Sport (VIN.......186377).
The LCD backlight on the dash mounted temperature control unit does not go on when the lights are turned on. The LCD display is working fine. But at night you can't read it because there is no backlight to illuminate the degrees, adjust the fan setting, etc. The little red LEDs for EC On/Off, recirculation On/Off, etc are all working.

The LCD backlight for the radio controls mounted below the temp control work fine.
Appreciate any suggestions to fix this problem.

Slim Jim