Issue with horn

I had the same issue with aftermarket steering wheel and horn button. If the new horn has two connectors, one side is hot and other is a ground. The issue I had was the button itself. When pressed, it didn't allow the contact pad that is attached to the button to make complete contact. I think that is why your test light is flashing when pressed. I took a die grinder and took about a 1/16 off one side and had to take close to 1/8 off other side to get it flat. But, if it worked before....I would start with the wire from the bulkhead to the horn to verify a good connection and not burnt or broken. Giving you a partial connection. That is the easiest. Could be an old horn relay too.
~Brian