Brake light rear brake light works fine when pressing rear brake pedal....but rear brake light does not work when pressing front handle brake

SOURCE: rear tailight blinkers not flashing

You have a short. The longer you ride the worse it will get from vibration wearing off protective coating. Trace wires or finding curcuit and running new ones may be easier.

SOURCE: 2005 Yamaha VStar 1100 brake light doesn't work

brake light switch is gone...easy fix.

SOURCE: rear brake light wont work when bake break is

Examine the rear brake pedal and linkage....there should be a light spring that pulls on the brake switch when you push the pedal down...possibly the spring broke and fell off.
If all is well...there should be a connector nearby for the switch...usually 2 wires....12 V power in on one, and power out to brake light on the other. With a multimeter, unhook connector and with key on...you should have 12 V on one of the two wires in the harness...if you don't...there is a harness issue.
If you have power to the switch, now test for continuity (ohms) across the unhooked switch wires ...should be open (no continuity ) with pedal up, and when you press down pedal...the switch should close and show very low ohms continuity. If you cannot get continuity...normally there is an adjustment to move3 the switch out further so the spring from the pedal pulls on the switch plunger more...if your are unable to get continuity by adjusting switch mount..replace switch..