Installs right into the housing with minor difficulty. It is an LED so it is sensitive to polarity. In one way, it can light up together with the headlight bulb. If you flip it, it will only work with the light dial is set to AUTO and the headlight is off (meaning there is enough light on the auto daytime sensor). I wish they have a fix for this so that it stays on all the time just like the halogen bulbs.