As you are likely aware, the three marker lamps at the top-rear of the trailer, mounted around the center line of the vehicle, are called "identification lights." Their purpose is to alert following drivers that the vehicle with these lights is a commercial vehicle wider than 80 inches. The regulations for mounting locations are, in Canada: at the top, and maybe lower if the door header is narrower than 25 mm; and in the U.S.: "as high as practicable."
For a period of time in the late 1990s, there was a U.S. interpretation that allowed lower mounting of these lamps, but it has since been clarified to require that all lamps are mounted as high as practicable. One case in which they can be mounted lower is for flatbed trailers that have no high structure; it is allowable to mount them in the rear sill of the trailers. With the development of very small LED identification lights, it becomes much easier to locate the lamps in the upper header of most van trailers and most trailers in general.
One other element of the law is that the mounting of lamps is often at the discretion of the vehicle OEM, who is responsible for compliance certification. This certification will be accepted by the NHTSA unless it is "clearly erroneous."
To comply with the spirit of the law, it is recommended that a trailer is designed so that the rear ID lamps can be mounted at a high level. This would avoid regulatory scrutiny, as well as any accident-related product liability issues. Truck-Lite is aware of issues that some trailer manufacturers have experienced with new trailers being held up at the U.S./Canada border due to the lower mounting of lamps, when, in the view of the inspector, the lamps could have been mounted higher.