Suddenly my 2003 GMC Envoy is turning it's rear brake ligh on several hours after the car is turned off. I make sure all light go off when I cut the car off then several hours later my brake light are back on.

Check for a missing pad that presses against the brake light switch. If it's missing, the lights will stay on. They deteriorate, over time, and disintegrate. They're the size of a Lifesavor. There may be a hole where the pad should be, causing the brake lights to stay on.