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StormySky : I just started having the same problem this week. It started out one morning and the DIC and radio display did not show up. Today I noticed the same thing happening, but the stereo played--but no display! The dash lights did not work when I turned on headlights either. I'll look tonight to see if the headlights, etc. come on. I just got it back from the dealer for the recall, replacing the top, and a rattle in the door! I don't want to take it back!
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In my car, there is no way that I have found to turn the instrument lights on manually. Covering the ambient light sensor will do it, but no switch will. I can turn the instrument lights off by switching the Auto Lights Function off as described above. When the instrument lights are on, the DIC and radio displays are dimmer than when they are off, even if the brightness control is set to maximum.
There is a time delay for the ambient light sensor to turn the instrument lights on or off, this minimizes the cycling on and off that would occur when going under bridges, etc. When I leave the garage in the morning my insrument lights stay on a lot longer than I would like for them to.
I have noticed that early morning and late evening light levels will turn the instrument lights on even though it is quite light.
It is quite possible that one or several problems are occurring with the lighting.:
1) If the displays are blank even after the car has been in direct sun for a period of time, the Auto Lights On system is turned on
and the headlights are not being turned on by it, the most likely culprit is the ambient light sensor. It could be a fuse, but in that case other things, like the radio, would not be working either. It could also be the BCM. It will not be the dimmer switch, as it is an input to the BCM, and is ignored when the Ambient Light Sensor is not calling for instrument lights to be on.
2) If the Auto Lights On system is turned on
and the ambient light sensor is properly controlling the headlights, the blank display is most likely caused by the dimmer control. It could be a fuse, but in that case other things, like the radio, would not be working either. It could also be the BCM.
3) One thing I have noticed is that the DIC and radio displays will go blank at a higher dimmer setting than the instrument lights.
The dimmer control is not a rheostat that directly controls the instrument lights. It is an input to the BCM. The BCM uses it to set the level of a PWM output that controls instrument light brightness. The DIC and radio display brighness are controlled by those devices, with the control signal sent over the low speed GMLAN.