Miles, is it possible the heavy cables to the solenoid are reversed? According to the wiring diagram, the horn ties to the hot side of the solenoid which goes directly to the battery. If the two large cables are reversed, the starter won't spin and the horn won't work.
Or, is it possible the over drive wire and the small solenoid wire are reversed? They look quite similar. If they're reversed it would not start but the horn should still work. Here's a picture