The relay is powered up and activated thanks to a small electrical current. When it is activated, it lets a current that is much bigger than the original one flow through other two outlets. It is used to control the headlights, the rear fog light, the electric fan, etc.
It's a tiny black wire going to the just underneath the hood latch and if you know wiring or have any idea one side is the signal that becoming from the steering wheel and then the other side is the earth or the ground and if you've done all that and you still can't get a horn it has to be something wrong with the steering wheel or the switch One way to try it is just go to those two black wires find the horn and put the signal wire to the battery if you don't have a momentary on switch that's a spring loaded switch and just touch that tiny little black wire that's easily broken and say a heavy snow region or ice or any of that stuff can just break those little wires
2017 Nissan frontier Horn don't work. I checked the horn fuse and relay and they were fine. Unplugged the horn and tested it with the battery and it worked. Plugged it back to the harness and not working?