I guess I will look into it. You are right, this has to be 9v or so, which brings us back to where the horn drive comes from. You can use the neutral for a DC common but that puts battery voltage on the signal wire to blow the horn. If this is not buffered by the remote detector (relay or solid state) it still depends on the supply coming from the signalling detector. It even takes a bit of current to pick a relay. Probably more than the horn.
