I got one of these Hella double horns and have had good results:

<<http://www.rallylights.com/hella/supertone_horn.asp>>

With careful fitting, I was able to leave the existing horn unchanged and
wire the existing horn ground (coming off the steering wheel) to a switch
that lets me change between the existing horn and the Hella horn. The
Hella is very loud and works well on the highway.

- Kevin