there are 2 types of horn buttons .
the 1st type ... grounds a contact to the handlebars ...when the button is pushed . ( finalizing a ground connection where power comes through the horn to the button )
the 2nd type ... contacts an input voltage to an output wire ... when the button is pushed ( power is fed to the horn , which is grounded )
the type of button determines ... where in the circuit ... the horn button can go .
Easy to tell the difference, 1st type has four wires (three for the dipper switch and one for the horn), and the 2nd type has five.
Hope this hasn't already been covered, but if the switch is of the 1st type, make sure you have an electrical path from the handlebar back to the "ground" side of the battery
(positive I assume).