I just purchased a 2005 trailblazer...what is the small knot in the middle of the dash where the windshield is?

That's a light sensor. It senses outside light. And when it gets so dark it will turn the headlights on for you...... If you want to see , lay something on it and start the truck and wait a few minutes and your headlights will come on and then uncover it and they will go out in a minute or two...........md

The light sensor that turns on your headlights automatically.

light sensor that turns on your headlights automatically