My Dad's 71 280 SE has the dash knob to switch the horn sounds! I used to play with it when I was young!
I have air horns on my Bug and I will tell you this from experience...
There will be a slight delay between hitting the horn and actually producing a sound, due to having the air compressor spool up first.
When mounting, aim the horns slightly downward, so that moisture can be expelled during heavy rains.
You want as little obstruction as possible in front of the horns or the most loudness...therefore, behind the radiator, for example, is not a good place.
On my Bug, they are mounted on the outside of the driver's side inner fenderwell. The surrounding fender provides the acoustics necessary to amplify the horns prior to the sound exiting the vehicle.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car