Ordered two sets of Hella Twin Sharp Tones. One is for my wifes Civic, the other is for the Fiesta. Have not installed them in the Fiesta, but today I installed a set into the Civic. Nice and loud. A relay is a must. I plan on installing them into the Fiesta when my Intercooler goes in. They were $18.00 for just the horns, $25.00 includes a relay.
If you have not ever wired a set of horns up.
1) If your factory installed horns have both negative and positive wires, tapping off of the positive terminal of the one of the stock horn wires, run a length wire that will reach the relay, run the positive to one side of the coil relay. You can run the negative power to the relay two ways. 1, find a bolt or screw with a good ground and run a wire from this to the negative side of the relay coil. Or connect the negative wire that went to the factory installed horn wire to the relay. This will trigger the relay when you press the horn button on the steering wheel. My wifes civic had only one wire going to it, it was the positive side. The horn bracket was the ground side. If this is the way the Fiesta is, run the positive wire to the coil side of the relay and for the negative side, just run a wire from the relay to a ring connector and attach it to a nearby bolt or screw that has a good ground
2) Tap a positive power source, battery or accessory off of the fuse box. If you go from the battery, splice in a fuse holder and a 5 amp fuse. Use that to go to each of the horns. Just run one wire and make a "Y" tap at the end, to plug into each horn.
3) Find a screw or bolt with a good ground nearby the horn location and using a ring connector with two wires, run them to the other connector on the horn.
4) Use a 3/8 plastic split conduit to protect the wire runs and zip tie the harness securely. Piece of cake.