Me. Rev. Stat. tit. 29-A, § 1908

On a vehicle 7 feet wide or wider, all rear lights, reflectors and signal lights must be within 12 inches of the extreme extension of the vehicle. On flat-body dump trucks, rear lights and signal lamps may be mounted on the rear of the frame. This section does not apply to unregistered farm tractors or to trailers with rear lights, reflectors and signal lights installed by the commercial manufacturer.