i love my Prius but the horn is laughable. I'm wondering if replacing it would affect my warranty. I'm thinking about something like this...
It's likely possible, if you can find the space to mount everything. There is a good amount of space just below the driver's headlight, in front of the wheel well. You can mount the compressor there. If the compressor pulls more amps than the stock horns, you should use a relay, and pull power directly from the fuse box auxiliary terminals in the back.
Also, regarding it affecting your warranty. It won't, unless you totally botch the installation, and even then, Toyota has to prove that it caused a warranted part to fail before denying a warranty claim.
Have you read through the long aftermarket horn thread on this forum (link in my signature below). Several members have installed wolo airhorns, and stebel nautilus horns.
The airhorns in the OP aren't too great. I had them for a while and the only way to mount them is with the horns pointing down in the stock location. They weren't very loud though so I'd actually recommend going with HELLA horns or something. They're louder and will fit with the stock location.
Is the difference between Hella and Fiamm that the later has the same terminal so no cutting is required? It appears the Hella requires a differant connector. Aw "Hell" !