You have a burned out bulb on the A/C console.

It's a dinky bulb (mine went out too), and you can get a replacement at your local Auto Zone or Pep Boys.

Carefully pull the center knob completely off, and you will see the bulb. You might need some tweezers or something to pull the bulb out, as it's nestled well in it's housing.

Replace the bulb, reattach the control knob and you're done!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car