When you say re-wire are you just wanting to mount the horn in a new location? If so just extend the original horn wire to the new location of the horn
If your talking about using the button in the middle of the momo steering wheel then you'll have to use a multi meter to identify which wire coming out of the connector attached to the stalk becomes live when the original horn button is pushed. Cut, extend and attach to one of the momo horn button terminals. Then either find a perminent live behind the fuse box, again using a multimeter, splice into it and run this feed to the other momo horn button terminal, or do the same from whichever wire goes live with the ignition up at the stalk connector.