If the bulb (as per Mikeyboy, and I think this is the most likely cause, too) is ok and you have had the bike a while and it's a new issue (new bike? recently played with wiring? playing with fuse box? lots of insulating tape covering dodgy joints? so much to ask), a few questions spring to mind, like, does it happen when the rear brake pedal is pressed? What happens when you select parking lights with ignition on/off? Any other wiring modifications not stated?