First would be to check the brake light to see if it's burned out or not .
Check the bulb socket for corrosion .
Locate the fuse box under the tank, it has 2, 8 amp fuses, check the fuse for security and cleanliness, you may want to bend the tabs that contact the fue, they are known to be problem on these bikes .
Replacing the fusce box with a 'blade' type fuse would be a good to get out of the way, so they don't give you problems when you're on the road .