The relay is probably the best place to start. Within 30 seconds or so you should know exactly which circuit is at fault. Disconnect relay and check the 3 connector terminals: one should be hot, one to horn, one to ground switch at column. If any one doesn't work, chase that circuit. If they all work, replace the relay. Sounds like it started when you took it apart so most likely in that area. If it is in the switch, as you suggested early on, you should be able to jumper the wire to ground (a test lamp will do) before the switch and make the horn operate.