The exciter (or primary) coil for ignition within the stator gets its ground through both brake light switches. Pull the brake lever(s) and the coil finds ground through the brake light bulb thus illuminating the bulb. If the bulb burns out then the engine will die UNLESS there is a resistor in the tail light housing that supplies a third path to ground. Not all C.E.V. tail light housings have this resistor. Electricity takes the path of least resistance, and thats why the bulb and resistor only work when the ground is broken at handle bars by brake switches(s).
You need to check ground going to tail light with a continuity tester. A bad ground to tail light will cause this.
Remember: the ignition supplies the current to light brake light bulb through ground. (edited)