The cost of replacing a car horn relay ranges from $15–$50, depending on the make and model of your car. Replacing the relay requires using a screwdriver to access the fuse box inside your car, which is located under the hood or under the dashboard. After removing the faulty horn relay, you must install a new one, reconnect any wires that were present in the old one, and replace any faulty wiring. Finally, test the new horn out to make sure it is working correctly before closing up your vehicle.