The horn relay is usually plugged inside the fuse panel of the car, and its location is marked on the panel ridge that encircles it. The schematic drawing in the owner's manual should indicate the location of the fuse panel.
The fuse panel is beneath the car's dashboard to the left of the steering column. Some cars might have a snap-on plate covering the fuse panel beneath the dashboard.
The car's main fuse panel is normally located under the hood on a wheel well or on the firewall of the vehicle. This panel also has a snap-on plate cover that the owner can remove using a screwdriver with flat tip.