Might I recommend a Whelen Alpha set up with 100w speaker...
A relay driven by the horn circuit and power is all you need
Single tone setting is perfectly adequate for most jobs. A quick press of the horn only sounds the factory horn due to the relay switching delay, but if a quick beep isn't enough press the button and hold it down and you get the full ARE YOU OPPOSING BLIND effect
Has the added flexibility to provide PA ability with a mic...