Logic tell me to connect a wire from the horn button, out the window, over the wing and to the horn.
If it goes "beep" then you have a break in the wire, if it goes 'clack' then you don't have enough current either (a) arriving at the horn or (b) going to earth (which needs a clean) or (c) the horn is faulty (they do rust up and die).
If (a) connect a wire directly from the battery to the horn switch.
If that works ... if not get a new switch.
(b) clean the earth, ie where the horn attaches
(c) get a new horn.
Do not be tempted to get a dual tone air horn or some other/similar, overly loud, monstrosity.
Unless you are (a) Italian or (b) a Redneck