Many have found that a common source of the problem with no brake/tail light is in the wiring harness under the fender that leads to the tail light unit. If the tire rubs the harness on full compression, it doesn't take long for it to wear through and separate the wires. It comes through a hole in the rear fender from under the seat on most bikes and follows through a channel to the light but sometimes is exposed enough to cause a problem area.
You will usually find the fuse blown for the lights also.