You don't need a relay--I did not and I use my horn a lot. In fact, for the first year, I had both the aftermarket and stock horn going at the same time with no issues (I use my horn quite a bit). However, for the set-up described above, one would certainly need a relay.A relay is needed for any horn more powerful than the stock horn. The stock horn can be left in place. Slip the electrical connections off the stock horn and use them to trigger the relay. I put a Stebel Nautilus air horn on the left side on a Pat Walsh bracket and a Fiamm (NAPA branded) low tone Freeway Blaster on the right side, bolted to a radiator mounting bolt. The left horn is for people merging into me from the left, and the right horn is for people considering entering the roundabout when I'm continuing around. Of course, both blow at the same time. A pair of Freeway Blasters (hi & lo tone) would have worked very well also.