alright,...i found information regarding antique horns of this type,
the capacitor in the horn is typically 15 uF, much larger value than the bypass capacitors found everywhere else in the truck.
the horn must vibrate within a tighter frequency range to allow the tone disk to resonate and produce more sound.
the capacitor in the circuit makes the operating frequency of the diaphragm and armature much less sensitive to a change in operating voltage (engine on vs off, headlights on, bad generator...etc)
-s
