I recently did a repair job on my horn, it had not been working for many years. But my problem was not exactly the same as yours. The horn contact is easily damaged by the process of removing the steering wheel. You might be able to make a new ferrule by using some parts found at your local hardware store (neighborhood store,not a warehouse store). Take a look at nylon spacers. Maybe you could find a brass screw that would fit into the end of a nylon spacer (make sure the head of the screw isn't wider than the diameter of the spacer). Maybe you could crimp a new ground wire onto the screw before inserting it into the spacer. A grinder would be handy to make parts fit. While you're at the hardware store get a new spring that will be the right size so that the contact stays pushed up against the horn ring. Or maybe you can find something else you can adapt.
You should also get a new rubber insulator washer. These disintigrate with time, and if it does not insulate properly you will end up with a continuous blasting horn. The washers are available from classic volvo stores such as Swedish Treasures.