HVAC controller display goes dim, then dark. HERE IS a fix for that.

[MarkHull]

A common problem where the HVAC controller digital display goes dim or dark.
The problem is that the surface mounted resistors on the circuit board have oxidized solder connections. Simply heating them up with a soldering iron(re-sweating the joints), resolves the issue. A new HVAC controller costs about $536.00 from the dealer. Here is a detailed link with step by step instructions, and photos. Also some links with info on people willing to fix it for a small fee.
I followed the instructions, and now my display actually lights up again. Saved a lot of money. By the way depending on the part number, most of the hvac controllers are not even available at the dealer. The dealer will "send it out" for repair, for some ungodly sum of money. If they do have it, the price is $536.00 for the part, and then of coarse labor. Here is the link
C5 Climate Control dim/dark display fix - Corvette Forum

[Vette_Dreaming]

I was just about ready to take my C5 to the dealer to have the HVAC controller fixed. I just happened to run across this thread. I think it took me longer to get the dash apart then it took to actually resolder the parts. Thanks to this article I now have a bright display.

[Junkman2008]

One thing to note. Sometimes, the LED's that illuminate the buttons will fail to work after this repair. The fix is to replace those bulbs. I usually do both when I do this repair.