What's the issue when headlights don't work on 2005 Chevrolet Trailblazer

My nephew put a new radio in, when he was finished, I had no headlights, the ac didn't blow out air and some other lights did not work as in the radio and inside lights.

Asked by for the 2005 Chevrolet Trailblazer
The simple answer, and probably the issue, is CHECK YOUR FUSES. Then, if they are all good, you need to look at a wiring diagram and see what he "might" have tapped into.