Brake warning can come from 2 places. There's a limit switch at the parkin brake pedal and a level switch at the master cylinder reservoir. Disconnect the plugs on both of these, and the light should be out. Now connect one of them, if the light is on you found the problem.

Light on with just the brake reservoir switch connected = low brake fluid or switch is stuck.

Light on with just the switch at the park brake connected = park brake not released fully or switch is stuck.