need 12 volts at 30 amps under hood

The factory horn wire is going to be your signal wire That's going to activate a relay wherever you put it in the relay will pick up 12 volts they off the battery with an ample size piece of wire which will probably match the wire that you're horn already has going to it and then off the relay when that wire gets energized it goes to the positive wire on your horn and energizes the horn all with the factory wire only thing being added is the relay that's going to open a contact that's going to allow you to feed that contact with a 30 amp sized wire and then power your 30 amp sized wire that goes right to your horn and now you have a blaring horn apparently we shall see. All your original horn wires going to do is open a relay what the relay opens to is the fat wire that you put on the battery or somewhere under the hood pick up near the fuse box probably and then the new horn wire plugs into the other side of the relay so when your factory horn wire is energized by you pushing the button it opens the relay lets the voltage from the fat wire you now have connected to one of the fat wires near the fuse box and opens that circuit and plays the horn on the 30 amp wire you'll see some fat wires up there around the fuse box you can probably tap your 30 amp horn off of without any problem your horns never going to be anything that's used continuously generally speaking