Battery powered horn [comes with a switch] can fit any bike

A battery powered horn kit that can be fitted to any bike. It includes the horn, a battery pack, a switch and a splitter. Orders ship within one to two days.

Running its own battery is the point. There is nothing to splice into the bike loom and no load on the 12v side, which is what makes it work on bikes that have no 12v supply at all. It is also the simple answer on a bike where the stock horn and its wiring have been deleted.