The 103 is wired as ext ignition ground, correct? Meaning, NC switches and brake light running off of ignition side ground. Some are gonna say you need that resistor thing by the back rack or one in your tail light, etc but:
1. you are getting ground from somewhere, since runs with levers pulled, light blown.
2. your brake light is wired into the wrong side of the coil
I just had a similar issue on a city bike, literally did the rev up/brake lever thing for 5 mins brake/throttle, On/off to try and blow it out, silly. Spent a day getting nowhere and gave up.