I noticed today that when I cut my bike on the tail light doesnt come on. When I pull the brake lever both tail light lights light up. If it helps I have a 99 cbr 600 f4. I tried replacing one of the bulbs that had a broken wire inside the bulb, but still didnt help anything. I checked the small fuse box by the battery under the seat and they all looked fine.

 

So you're saying you don't have running lights? Just brake lights?

 

Correct

 

I had the same problem on my 88 600. It was in my contact base on my ignition switch. i ended up modding the inside with a spring and it works. but in turn i lost power to other things. Honda wants 40 for a new contact bace so ill go that route. hope this helps you.