Maybe it's time to see if there is power on the wires? Trace the wires back and find out where they hook up. Worst case scenario, run a new wire from the stator output yellow wire to the tail light. Ground the tail light housing to complete the circuit. That will have it working any time the bike is running.
If you want a brake light, that function needs to be activted by a hydraulic switch on your rear (and front) brake systems. Get power to the hydraulic switches, then through them to the brake light.
