My little French four stroke Terrot has a non standard brake light switch fitted, it wouldn't have one as original fitment and was in place when I got the bike. However, it doesn't work. I took the rear light cover of and there is only one bulb, and it's festoon type, exactly as it should be, could it be a twin filament stop and tail type? The bulb itself lights brightly on side light but as it is an opaque one I can't see inside to see if a filament has gone... Any ideas please