One problem related to fog light switch has been reported for the 2003 GMC Envoy. The most recently reported issues are listed below.
Riding on freeway and headlights suddenly went off in dark area. Cut on fog lamps and headlights came back on after couple miles of travel. They have not went off yet again but would be dangerous on curve road. Problem maybe in turn signal control. Cruise went out day before and came back. All located in same switch. Called GMC and told them of problem. Have not went off again yet. Talked to dealer and they want to see it with problem.