I have the Hella 3 air horn set up on mine.  The kit includes a relay, three
horns, a compressor and the tubing.  I got mine form New Dimensions, <
http://www.newdimensions.com > at a show last year, the horns are on their
site also, in 2 or 3 horn kits.  The sound in fairly high pitched and
extremely loud, the sound is European (if there is such a thing), i.e. not
like a train.  I've had them installed for about six months, no failures.
The compressor is mounted where the old horn was, on the same bracket, it is
shielded by the spoiler.  The horns are right behind it between the frame
rails to the right on the spare tire, right as in passenger side.

dave
www.251.org