Recently upgraded my 2014 Prius with a pair of Hella horns. I did a lot of Internet search for a viable wiring diagram. Could not locate a single working diagram. Decided to rely on my own engineering knowledge, though aeronautical, to come up with one. Diagram posted here works absolutely fine. Good luck on your project!
Are you still following this? Just FYI: These horns are designed for continuous use. Typically used by the emergency services in Germany, for the bee-par, bee-par, bee-par ('get outa my way') sirens. Designed for a narrow but far reaching noise envelope. Excellent horns.
