Yep, 6 factory horns and the car sounds like a train. I used them today on a women in a tahoe who didnt seem to care that my lane was merging into hers. She just kept creeping along side of me while the lane to her left was completely devoid of traffic for about a mile. Needless to say, she merged into the left lane after I held down my horn.
I recommend wiring them in parallel. In series your going to have voltage drop because each horn, basically an electromagnet, has a certain resistance. I put 6 horns on mine in a big cluster and stopped at 6 so I dont draw to much current through the horn contact however im considering adding 6 more with the aid of a relay and a direct connection to the battery
I'm thinking of adding two under my rear bumper for future soccer moms!
Their so easy to wire in and can mount almost anywhere and with the aid of a relay there's really no limit to how loud you want it to be. Lol