The a/c pushbutton panel has a small illumination bulb mounted to the outer case. You'll need to remove the a/c panel using special "radio keys" (that's what I call them anyway) in the slots on both sides of the front of the a/c panel to disengage the retainers. The "radio keys" are available from MB dealers, or may be available elsewhere also.
Once the a/c panel is pulled out of the slot, you'll see a small round white bulb socket- turn it 90 degrees using small pliers & pull out to remove. I believe it's a 1.2 watt bulb, but double check to be sure.