Actually.. I was in a shop yesterday and came across a compressorless dual air horn by hella. I was thinking about getting it because I drive the highway a lot (mostly between school and home) and I had two close calls where the guy coming into my lane couldnt see or hear me with the stock horn and a louder horn would of saved me. A lot of people dont realize our horns are ineffective on the highway, as in- you cant hear it at all. I can hardly hear it when I'm in the car, doubt someone else can right next to you, even with their window down. Here is the part number: 3FH 007 424-801 (12v)
UPC: 4 082300 070538
This will take a minute to load, check page 5:
http://www.hella.co.za/DownLoads/Hella%20Accecories.pdf
It was 30 bucks at a shop around here.