One problem related to fog light switch has been reported for the 2005 GMC Envoy. The most recently reported issues are listed below.
I was driving on a curvy 2 lane road in the country and the sun was just setting and I couldn't get any lights on my car. My lights are automatic and I just assumed that I had bumped a button and accidently turned them off, so I started turning the switch to turn them on and it wasn't working. I almost drove off the road several times trying to get some light on. I then hit a section where tree lined curved section that was pretty dark. There had been no shoulder or anywhere to pull over there or for several miles. I could get bights lights, but no dim. I ended up driving with my fog lights, unknowingly at the time. We had just replace the bulbs 5 months before but we tried new bulbs and fuses to see if this would fix the problem. Unfortunately, it didn't. I have noticed a recall on lights for the Envoy years 06-07 for what seems like the same reason and mine is an 05.