Tools needed. 12mm spanner, 10mm spanner, something to crimp or solder the connectors, vise mole grips or pliers to bend the bracket, small screwdriver or blade to pop up the clips. Some grease.
The Prius has a horn relay and 10A fuse. The original horns are about as loud as a scooter horn and on test draw just over a 3 amps each. The FIAMM's I replaced them with draw 4.5 amps each so should be ok as a straight swap. You are safe to up the fuse rating to 15amp but it shouldn't be necessary.
I fitted the FIAMM AM80SX high and low pair, there is little in it between the AM80S combination sound level wise, but they are a little smaller. FIAMM are superior to the offerings from Hella, Bosch, PIAA and Stebel. Stebel's Nautilus air horn IS loud but that draws 20 amps and needs a separate relay, and there are loads of look a likes out there, beware. I have bought all of the other makes and they were either binned or passed on, stick with FIAMM.
You need to remove the black plastic radiator cover held in with 7 plastic pop up clips. Push a small screwdriver or blade down under the centre cap, it will pop up and can be lifted out. Once all removed the cover lifts up. The plastic clips push back into the hole and the centre cap pushes back down, don't loose them!
The factory horns are easy to see, held in place by a 12mm nut, a ratchet spanner is the best tool to remove them with, when you have loosened them remove the connector. You will see a square hole on the left side looking down on them, push it in and pull the bottom of the plug down. Now remove the nut being careful not to drop it or the horn, as the radiator is close. The factory horn has a small extension bolted to it, leave it on the old horn.
The original horns use a locking spade connector (6.3mm) and you will need two short leads make up of a male to female spade (red insulated spades are ideal size for this) for the live feed and Female to ring terminal for the return (the ring needs to fit the 12mm bolt) Both leads about 7 inches long.( If you get horns with only one terminal you do not need the spade to ring lead) The FIAMM's come with a straight bracket, this needs bending into a question mark ? shape (see picture) this will allow the new horn to sit in a similar position and pointed slightly down so any water will run out.
Bend the bracket and test fit, when you are satisfied, fit your leads to the horns, grease the connectors, they sit in a lot of weather down there. ACF50 or Waxol or whatever. Also grease the connector on the car and your bolts and nuts.
Now bolt your horns to the original car fixing using the 12mm nut,don't forget to put the ring connector on the nut, place the horn, put the bolt in the bracket and offer it up. Three hands help here!! Get the bolt threaded in and before you tighten it push your connector (male spade) into the car connector the male will just push in, make sure it goes all the way in and no part of the spade is bare connector or wire, if it touches the chassis member it will short out. You can zip tie the wire but if it's not too long it will sit in mid air. Tighten up and test, if ok do the other. All working remove any spanners and other tools (count them in count them out), they will rattle and bounce around till they drop out doing all sorts of damage.
1st picture Fiamm AM80SX left AM80S right 2nd Original horn and AM80SX bent bracket. You are aiming to get the new horn about the same height and pointing down a little. This was bent up a little more.