Maarkr: I use synth produced car horn sounds in my music sometimes. It would be musically abstract and not the sound of a actual car horn and so it depends on how you play it. I take a synth like harmor [or whatever you prefer] preset to a basic sound with a slight oscillation and not too sawy. Pad it with another synth capable of producing a mild detune offset. I use Novakill's free NitrousPM for that. Generally, a "C" + "E" chord[next one up from middle c] will give the right effect. It works for me but it may not be the same as what works for your music. Still, I think producing a certain kind of detuned sound in a pad with another synth is the general idea.