It sounds like somethingay have damaged the wiring, or perhaps some modifications were done incorrectly. You'll need to identify the wires to the reverse light and the brakes, and start tracing them from the rear to the front to see where the issue is. If they are being pinched by something conductive then when you hit the brakes the power is also sent to the reverse light. Though that one isn't likely as it would most likely also short out and blow a fuse...