I figured out which wire goes to the original horn. The wire is color coded (use the shop manual to see the colors). The horn wire parallels the radiator. I striped the wire and spliced in (twisted, soldered and covered in shrink casing) an additional Honda Pilot horn. So now, both horns sound at the same time. It is plenty loud. When the horn is used on the freeway and anyone drifts into your lane, they immediately correct their direction and the collision is avoided.
I mounted the new horn right in front of the radiator between the hood and front bumper and screwed to the upper radiator mount. When the horn is used, the full volume is facing forward.
I didn't have to modify the horn relay or take off the bumper to remove the old horn. It may not be the best way to install a new wire to the new horn, but I use my horn everyday at least a few times(SF Bay Area traffic commands a loud horn).
My installation is over 200,000 miles in use now without failure