Im a fan of cheap and easy, and have written a few threads of similar titles. While building my Lifan 125cc I began to think this bike will draw more scrunity from local Enforcement, so maybe it should have a brake light. I found a LED trailer clearance light on ebay for $3, it super bright. I mounted it in a well seen location below the seat. It came to me to use the brakes as a switch. I added a throttle stop to the cable between the levers. I then mounted a contact screw to a piece of stiffer but flexible plastic with a screw for a contact. Simply make the lead to the brake always on and hook the ground to the screw with a ring terminal. Adjust the throttle stop to hit the screw about 1/4" before the brakes seat. It makes positive contact and the plastic flexes increasing contact pressure. This is on V brakes but probably can be adapted to most brakes where you have access to cable travel. There is no danger of a short if the wire gets loose as it is a ground, light just will come on.