Wire is lower down the column about 2/3 thirds down and on top. It can get ratty over time and it can be fixed. But still a good place to start. The horn is grounded through the button and the column so make sure it the column is snug in the brace mount at the bottom of the dash. Two bolts and a clamp. Then run a wire from there to ground for a test. If that fixes it then clean the place where it is mounted and the clamp. It could be as simple as that.
