Hey guys,
i've got a 99 cbr 600f4, both brake lights work and turn on with the brakes. but neither is on just as a tail light. why is this?

thanks

 

Did you check the bulbs? The tail and brake are different filaments.

 

As 74Demon suggested, check the filaments first. You can check for continuity with a multi meter by touching one of the soldered pips at the base of the bulb and touch the other lead to the outside of the bulb. If bulbs check out ok, check for voltage on the inside of the bulb holders on each of the supply wires with your meter connected to the wire and earth. If you are getting voltage at both connections inside the bulb holder(s) then you could try squirting some WD 40 into the holders and pushing on the connector inside to free it up. They sometimes stick and don't connect properly.