How do you cut off the brake light on a 2003 Mercury Mountaineer?

The brake light is controlled by a little button switch that's mounted high on the brake pedal lever. If you follow the brake pedal upwards with your fingers, you'll feel the button.

It has a spring in the button that pushes the button back out when you release the brake pedal. If the spring jams, the brake lights stay on.

Replace the button switch.