What is the illumination wire for the kenwood KDC-138?

That model is not equipped with an illumination input. Illumination wires will dim the head unit's display when you turn your car's lights on, but they're usually only found on higher-end models. The KDC-138 is an entry-level unit, and lacks the auto-dimming feature. Kenwood uses orange/white for the illumination input on their head units.