I have a RAID Dino wheel and hub that has a horn button with just one wire. The circuit is closed when the copper cap of the button contacts the copper center shown here.

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How do I convert it to a two wire connection for a new Porsche center button from a MOMO wheel?

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Can this be done simply?

I have a Dino wheel with a Porsche horn button as well. If I remember correctly my button just had one tab that connected to the cars single wire. I think one of the wires on your setup needs to go to the cars wire and the other one needs to be grounded. I assume the one with the shield goes to the cars wire. Easy enough to test it just connect the wire with the shield and touch the other one to ground and press the button. After looking at your button photo again it could be the other way around. I would make up a pig tail with a spade connector and ground it to the back of one of the wheel bolts to pick up a ground and run the other end to a male connector to connect to the female wire on your button.

One wire goes to ground one to the original horn wire

Can I ground to the hub or do I have to find a way down the steering column under the dash?

The hub is already the ground.

One wire from your button goes to ground and the other wire goes to the car horn wire.

Thanks, i'll let you know how it goes.