Wide band or dual band jamming could be the ticket. knock out cell phones, fm radio reception. Now if you could just send out EMI to fuss up the pre-amp signal you'd be set. Lower freq are harder to dissipate, so more of the sound makes it through the car. But then you're competing with the subwoofers, a couple thousand watts in a confined space is hard to top even with train horns. You'll need a couple of notes (freqs) to get a range that will attract attention.

I think this is probably the problem you're having with the electronic horn. It's producing notes in the cheaper speaker range. So you're having to compete with car's audio and the insulation. You may want to look at very high frequency ear splitters or ultra low shaker type noises, or both for the double wammy.