http://www.eastmarinedrive.com/audio/2003/horn.html

Have a look. its sort of weird. Reckon this could be implemented in a car.

It has been done before although it’s not the ideal acoustic environment for the horn style loudspeakers.
Horns are really only implemented into very high fidelity systems which cost enough to pay a not so shy deposit on a house.
Cheap Horns can be had; although they usually are not the best sounding considering the coin invested in them; nor would they be practical for in car applications.
Stick to the conventional speaker driver for good quality sound for great price; just buy finest your funds will stretch for.

Horns are used by some people in the car environment. There are quite a few drawbacks not the least of which is there size and where to mount them.

Unless you get really good ones they also tend to sound nasily IMO.

In PA use i use them all the time in fact it is the only way to throw high frequencies out to match mid's and low's.