first things first, start with the bulb" see if the bulb has two elements, , one of them if for the tail light and the other is for the brake light, as on my bike, if its just has the one bulb in the back" take it out and look at the bulb , and see if one of the elements wires have blowin" if so thats why the is wont come on, replace it with new one , hope that helps.
mick
April 2008