The controller's cpu monitors the 5v on the brake signal wire. The default is on, as that wire is held at 5v by a pull-up resistor. When the CPU sees 0v on that wire, it sends the code for that symbol to the LCD's CPU, which puts it on the display. That 5v brake signal wire would only be 0v if it's shorted to ground, either by a brake switch or a short in the cable attached to the controller, so that's where you need to look.