They keyboard backlight works fine on my rMBP 15". I've done a fresh reinstall a few times because I'm a nerd, and I always have to turn the backlight down because it's always on bright on first boot. I just did a fresh install (erase Macintosh HD then reinstall OS X), and the backlight didn't come on. When I try to turn it up or down, a greyed out backlight icon pops up (the same one as normal, but its greyed out instead of white), and it has a 'prohibited' icon below it (circle with a line through it). How do I get it back in working order?