My wife had the misfortune of the horn sounding each time the signal was turned on. Solved problem by not using signal… However, the horn still went off after the truck had been turned off for an extended period of time (long enough for her to leave and not hear the horn until returning a couple hours later). She was able to stop the horn by placing the vehicle in neutral (manual transmission). I have not been able to reproduce the problem. I thought it may be the “relay integration”, but after removing this from the vehicle the signal does not work, but the horn still works. Is there a relay, switch, etc. that controls both of these devices?
I think the trouble may be with the wiring inside the steering column. The horn is usually activated by making a ground connection when the horn button is pushed. Check for a problem near the wiring for the turn signal switch or where the wiring comes out of the column near the firewall. There may be a relay for the horn and if so pulling it out should disable the horn if you need to do that.