Apologies for the criticism Graham but there seems to be a degree of re-inventing the wheel here.
Wonder why you felt the existing horn wiring was not up to the task? It worked fine for me when I dispensed with the Roadsters twin two tone horns and fitted Siebel twin air horn (which likely draw more current than your Fiamm's) although I added a ground switched relay and upped the existing horn circuit fuse to 20 amps. Note the factory wiring provides power to horn circuit only when ignition on or when first accessory position is selected. Use of this would have saved your fireworks, blown fuses etc.
It appears you have dumped the horn push in favour of a momentary switch with live connections
. Was this because for whatever reason you couldn't wire the relay coil to trigger from the ground provided by the horn push as this is how it should have been done.
Not sure why you have posted relay diagrams showing changeover circuits employed, noting that top one is wrong, strictly speaking, in showing reversed relay coil pin connections
.
Happy to provide you with instructions on wiring your horns correctly if you decide to simplify, just send me a PM. It may save your car going up in smoke!
