"position" light = parking lamps in the USA.
Turn your headlight switch to the center position, with the key off. All of the exterior lights that turn on are considered "position" bulbs. The 4 rear taillamp bulbs are monitored by the body computer, and one of these is usually the cause of the message. It does not have to completely fail to trigger the message. If the filament resistance increases near the end of bulb life, the computer will detect it.