LOL!
THAT would be interesting.
There is a way to hook an outdoor bullhorn speaker cone to your car stereo.
Would work the same way as hooking your CB to also do PA like the cop car light bar does.
Flip a switch and sound goes out through the PA speaker.
Now that I have a different car whose horn I don't want to alter, I bought canned horn - essentially a horn on an aerosol can - and it sounds like a steamship! I haven't tried it on the road yet but it worked great evacuating the house!