Iowa Code Title VIII. Transportation [Chs. 306-330B] § 321.388. Illuminating plates

Either the rear lamp or a separate lamp shall be so constructed and placed as to illuminate with a white light the rear registration plate and render it clearly legible from a distance of fifty feet to the rear. When the rear registration plate is illuminated by an electric lamp other than the required rear lamp, the two lamps shall be turned on or off only by the same control switch at all times when headlamps are lighted. This section does not apply to commercial vehicles engaged exclusively in intrastate commerce that are dump trucks or that are used exclusively for the movement of construction materials and equipment to and from construction projects.