If it's anything like the standard Corolla Hatch, this procedure may work (lots of good details, including suggestions on which horns to buy). Apparently the normal CHB even has a spot for the second horn.
www.toyotanation.com
Horn Upgrade
Like most Japanese cars, the Corolla has a rather unfortunate, wimpy horn. First thing I did when I got my Miata years ago was to install some Hella Supertones: https://www.amazon.com/HELLA-Protective-3AG-003-399-801/dp/B000CRZXPI. I love it when I hit the horn and people look at me wondering...
www.toyotanation.com
