Did you change the relay out? A lot of time the relays stick when they are going bad. Try tapping the relay while pushing the horn button. If that works, change the relay.
Also, clean and tighten your grounds. If that doesn’t work, it is most likely your horn button, assuming the relay is good and correctly installed. I haven’t had the C14 switch housing apart. Not sure if you can clean the contacts on the switch or if it would have to be replaced.
You could always bypass the relay by disconnecting it and touching the connections together. Since you know you have power at the stock horn, I’m assuming you at least have a test light. Do you have power going into the relay from the stock horn? From the battery for the aftermarket horn? Do you have power going out of the relay on both sides when the horn button is pressed?