The Power Call was never designed to be used this way, but it you don't mind altering the siren a little bit, it can be done. You would have to run a wire from the center momentary switch to the outside of the unit. From there, you would have to wire a Bosch relay both to the horn switch and then to the wire coming from the siren. The big thing is to figure out if your horn is positive or negative switching. Once you know this, you can wire the relay for proper operation.