One of the switches, on the handle bar, or foot pedal is closed all the time. Disconnect one at a time and determine which one it is. It may just need an adjustment to keep it from completing the circuit. If not, replace the switch. Sometimes the brake filiment (inside the bulb) will break and fall over onto the tail-light filiment. This will cause the tail-light to light brightly, as if the brake switch is closed in. Try a bulb first.