Motor vehicles with registration plates are the only ones required to have a rear registration plate light. This is because these vehicles must have a visible registration plate at the rear, and the light helps make this plate readable in low-light conditions.

- **Automobiles**: They must have two registration plates, one at the front and one at the rear. The rear plate light is necessary for the rear registration plate to be visible.

- **Motorcycles and mopeds**: They only have one registration plate at the rear, so they also need the light to ensure its visibility.

- **Trailers and semi-trailers**: If they can carry more than 750 kilos, they must have their own registration plate and that of the towing vehicle. The rear plate light is important for both registration plates to be visible.

Vehicles that are not registered, such as some lighter trailers, do not have this obligation, as they do not have their own registration plate that needs to be illuminated. Therefore, only registered motor vehicles must have the rear registration plate light.