BLUF: License plates must be mounted horizontally with expiration year in upper right. Lights other than shades between red and amber for the rear or shades between white and amber for the front marker/signal lights and headlights are prohibited.
I know a lot of people mount plates vertically on the rear of a trailer fender.
From IGA:
"Requires that a license plate must be displayed in a horizontal and upright position that displays the registration expiration year in the upper right corner. Requires that a renewal sticker for a license plate must be securely affixed in the upper right corner of the license plate covering the previous registration expiration year. Provides that a trailer of less than 3,000 pounds gross weight is not required to be equipped with brakes. Specifies that head lamps on motor vehicles, motorcycles, and motor driven cycles may display only white or amber light. Requires that motor vehicles except for Motor vehicle safety. Requires that a license plate must be displayed in a horizontal and upright position that displays the registration expiration year in the upper right corner. Requires that a renewal sticker for a license plate must be securely affixed in the upper right corner of the license plate covering the previous registration expiration year. Provides that a trailer of less than 3,000 pounds gross weight is not required to be equipped with brakes. Specifies that head lamps on motor vehicles, motorcycles, and motor driven cycles may display only white or amber light. Requires that motor vehicles except for motorcycles, motor vehicles manufactured before January 1, 1956, and motor driven cycles must be equipped with two stoplights. Specifies that: (1) stop lamps on the rear of a vehicle must be red; and (2) signal lamps on the rear of a vehicle must display only red or amber light or any shade of color between red and amber. Specifies that signal lamps showing to the front of a vehicle must display only white or amber light or any shade of color between white and amber."