When a driver presses down on the brake pedal the brake lights on the back of the car shine brightly to warn following drivers. This has not always been the case, as the brake light wasn't used on American cars until the mid 1920's when it was first used on the Ford Model T. The high-mounted third brake light, which is now on all cars, wasn't invented until 1974.
The brake light is illuminated when the contacts in the stop light switch, located under the dashboard, are closed. When the driver steps on the brake the plunger of the stop light switch is allowed to extend from the body of the switch. This closes the contacts allowing 12-volt power to go to the brake light bulbs. Power then flows through the bulbs to the socket ground, causing the bulbs to light up.
In addition to controlling the brake lights, the stoplight switch is also used to disengage the cruise control. The stoplight switch has a second set of contacts which are normally closed but open when the pedal is depressed. Opening these contacts stop the flow of current to the cruise control unit, shutting it off.
