If the horns are made upon the default BIS "CarHorn" types then ya can't get the AI to use it m8
this is due to the type of 'weapon' we're talking about; the carhorns don't use any ammo ( = no mag's at all exists, the sound you hear is the "drySound" ), and thus: the AI thinks he's outta mags and won't fire the horn.
If you noted this, the AI never used horns in the orig. BIS missions; the only time I recall having heard a carhorn in a BIS mission is in the RES campaign where it not a "carhorn" but a custom sound fired via scripts/triggers
Hope this info. won't kill yer mission idea completely, m8