Air horn

Hi all, I just installed an air horn on my motorcycle and it's working okay but the old horn isn't working along with it. I basically stuck a relay between one of the two wires going to the original horn, and then used that relay to switch a circuit for the air horn compressor which is wired to the main battery with a 20 amp fuse. However even before i connected the compressor (ie, i just put the relay in), the old horn would not work but I could hear the relay "click" when i pressed the horn button, just no sound. I tried moving the relay between the other wire going to the horn, but same effect. I don't really know what I'm doing to be honest, but I do have a wiring diagram of the bike if that'd help? I want both to sound at once just because the air horn takes a split second to come on and I want instant noise :-)

I really have no idea what I'm doing, do I?