Joined: Mar 2008
Location: Prairie Village
If it slowly got worse, it might be the the horn itself dying/dead.
Though just to double check, using a multimeter, unplug the wiring from the horn and probe the connections while someone else presses the horn on the steering wheel. You should have 12 to 14 volts on the wires. If theres no power, could be the relay, fuse, wiring or ground.