Hello, this question seems to have been asked on every classic car forum at some stage, with various answers. There are lots of posts on here, again with various answers, perhaps search Nardi, and see what it throws up
I do know that you need the correct hub for your car, so assuming you have, all i know is that the ground (brown) wire connects to the black/yellow/rose wire, through the horn push connections. If you only have one connector on the horn, perhaps the ground is already connected on one side of the horn contact, through the hub somehow, but i'm only guessing
If so all you need to do is solder a spade connector onto the black/yellow/rose wire and connect that to the horn and then try the horn push, perhaps the separate ground wire isn't required on the single connection Nardi horn push. You said the horns do work, so the +ve supply to the horns is good, there is no +ve at the horn push, you're just switching a ground through the push, so you can't do any harm trying
Hopefully someone who has already managed to get their horn working will comment
