The relay in an HID (high intensity discharge) light kit does two things: it provides the bulk of the power to ignite the Xenon bulbs and it protects the electrical system of the vehicle by draining very little power from the battery. Aftermarket relays often are made up of a wiring harness containing the plugs and wires to the respective connection points. This is a quick and simple relay installation method. HID kits sometimes come with relays, and installation is part of the kit installation, involving the same wiring as the aftermarket relays.
Unravel the relay harness wires. There are two ballast plugs, and they are located on opposite ends of the relay assembly. The plug that is closer to the positive and negative battery wires goes to the ballast closest to the battery; the longer plug connects to the second ballast.
Remove the plug connected to the ballast and install the relay plug in its place. Remove the plug from the second ballast and insert the plug from the relay.
Make the connections to the positive and negative leads on the battery. The negative/ground wire of the harness will bolt to the tightening screw of the negative battery terminal; the positive/hot wire will connect to the tightening screw of the positive battery terminal.
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Plug the spade wires into one of the HID bulb connectors. Disconnect the harness that goes directly to the HID bulb and plug the spade connectors directly into the bulb. You need to make this connection only to one headlight.
The headlight connection provides the signal to the relay to draw power from the battery and allow the ballasts to power up on both sides. The relay is used whenever the vehicle is not providing enough voltage for the Xenon bulbs to ignite
