Any horn other than stock I would absolutely install a relay. Taking the electrical load off the existing circuit eliminates issues with some aging 
fragile parts like the horn ring under the steering wheel. I personally 
ran a heavy gauge wire to a fuse box that I placed behind the glove 
box. This fusebox also has provisions for 6 relays. I use to to power
the headlights,the new horn,and my seat heaters. I have one spare
spot that I plan to use for a laser to zap drivers that won't get out of my 
way. (Just kidding though I wish it was true)
Even if you plan to just use existing wiring for power a relay is needed
to be safe.
 I think Bell industries shared some of his information on now 
he did it.   I will search and see if I can find my wiring diagram that I made. We are in the middle of some major work on the house so
everything is moved all around and in big piles. 

Bill