use the original positive supply wire to the horns, and switch the negative from the relay, you can also wire it through the ignition switch that the horn cannot be sounded until the ignition is on.
take a bosch 30A cube relay and attach the wires as
85 = ignition power
86 = horn trigger ground from steering wheel
87 = solid ground connection
30 = horn negative terminal.
if you are using a denso relay then the diagram shows a coil and a bridge -the coil points are 85 and 86 and the bridge are the remaining 2.