The laws regarding the use of a so-called third brake light vary from state to state in the U.S. The third brake light refers to the light located at the bottom middle of the rear window.
Some states require all motor vehicles to be equipped with a third brake light. In some cases, the legislation is long standing. For example, the Code of Maryland Regulations Section 11.14.02.10 has required state inspectors to fail safety checks on vehicles without a functioning third brake light since 1985.
In other states, the matter is dealt with in broader legislation. For example, in California, Vehicle Code 25252 requires that all lighting equipment of any kind that is installed on a vehicle be in functional condition at all times.