My brake light switch failed when it was fairly new. After the bike heated up, the pressure switch would stick in the on position...rendering the brake light useless for warning others.
I bought a new switch and replaced it myself. It's in a difficult location and you have to remove the shield and brake line to replace it. If I had a lift it would have been easier, but I proved it can be changed sitting on the kickstand.
Some have had electrical connection problems, but mine was definitely in the switch itself. Here's the part numbers you need, along with fresh brake fluid to bleed the rear brakes.
http://vrcc.photostash.com/vrcc_6897/DSCF0930.JPG