Let there be no more assumptions. In TN, the law is as states:
Actual laws regarding TN headlights and taillights...
Tenn. Code § 55-9-402
(b) (1) Every motor vehicle shall be equipped with two (2) red tail lamps and two (2) red stoplights on
the rear of the vehicle, and one (1) tail lamp and one (1) stoplight shall be on each side, except that
passenger cars manufactured or assembled prior to January 1, 1939, trucks manufactured
or assembled prior to January 1, 1968, and motorcycles and motor-driven cycles shall have at least one
(1) red tail lamp and one (1) red stoplight. No non-emergency vehicle shall operate or install emergency
flashing light systems such as strobe, wig-wag, or other flashing lights in tail light lamp, stoplight area, or
factory installed emergency flasher and backup light area; provided, however, that the foregoing
prohibition shall not apply to the utilization of a continuously flashing light system. For the purposes of
this part, "continuously flashing light system" means a brake light system in which the brake lamp pulses
rapidly for no more than five (5) seconds when the brake is applied, and then converts to a continuous
light as a normal brake lamp until the time that the brake is released.
(g) A violation of this section is a Class C misdemeanor.
sooo there you go. no more questions of legality in TN. As for other states.... it's pretty much the same.