A relay-isolation harness is a good investment if you're planning to pull a trailer. This setup allows you to drive a set of trailer lights (standard bulbs or LEDs) without affecting or loading the bike's signal circuits. In the event that a short develops in the trailer wiring (typically in the plug area), an inline fuse in the relay package protects both the bike and trailer. Your bike's lights will remain unaffected and continue to work.
Trailer lights can be wired in one of two ways:
4-wire: Brake and turn signals are combined. Usually the plug on the trailer is a "flat four"
5-wire: In this setup, the trailer has a separate set of lights that act as the brake lights. Turn signals appear on a separate pair of lights.
Choose the universal kit that matches the way your trailer lights are set up. If you're not sure, get the 4-wire kit. It can be used for both 4 and 5 wire applications.