i just installed a new horn for my dad's civic about 2-3 weeks ago. if i can remember correctly, it's ok to use the existing wire. the horn should be 12v. if it says OEM somewhere, then it should work. if it doesn't, don't worry, you'll just get a blown fuse if you put it on.
to my experience, when i first put it on, the horn fuse blew. i replaced it and it works. i don't know if it's gonna happen to you. just take out the battery terminal and install the horn. everything should be fine.
