When I purchased my 99 Z-71 it was the first year for the auto lights, which I think is cool. But now the Av's have a pair of sensors, not one but two. Couldn't find anything in the manual. Somebody throw me a bone here!!
The second one is a sun-sensor for the auto-climate. When it sees lots of bright sun, and it's hot outside, and it's hot inside, and you've ask for cool air, it'll send more cool air to the top vents. If it doesn't see the bright sun...then it'll send less cool air on your face.