I just did the opposite and have a non fog switch sitting on my dresser, let me think of the install.
Ok, it's really simple, you first have to take out the vent above the switch, just put a butter knife under it and pry out (read focushacks.com for more specifics, I'm not gonna be responsable for any plastic snapping). You can then unplug the switch and there are screws there to take care of. I'm not sure if you have to remove some of the interior piece that sorrounds the switch to remove and additional screws, I need someone to verify this all with me. The it's a matter of pushing it out and swapping the new one in. When you know figure out where everything is it will be like a 20 minute job.