The diode kit is a lighting kit for your towed vehicle this hooks to the wires for your taillights and gets electricity to the lights for use in brake light and turn signal indication, while making sure that the system of your car doesn't get any electrical signals while towing. This is only recommended when your taillight housing doesn't have room for an extra set of bulbs with the bulb and socket kit because it is more complicated than the bulb and socket kit.