To be clear, if you are buying this for a car you will not need an additional wire or harness. A car or truck has enough power to plug directly into the existing wiring. It supports single or two wire installations. The horn comes with a relay. If you are buying for a motorcycle you will need a wiring harness not a relay as it comes with the horn. I bought it for my Jeep, I already have two horns in JK but this one is louder. It was a close landing.
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