Here'* a neat product (semi-concerning this thread), and it was in part developed by my college here in Ames, IA! http://www.whispering-windows.com/
You attach two of these modules (about the size of a chew cannister) to far sides of glass and the glass becomes a speaker! Neat or what?! I have yet to find a practical stereo app for a car (other than a really sweet center channel for a 5.1 setup), but they're sorta neat to know about.
And expensive, too, lol. Almost $1800 for a single channel! They've got 'em installed at Jack Trice Stadium and are really something to hear in action. In fact, on large panes of glass, they're really not noticeable, as you can put the modules in the corners by the floor.
I just noticed they also make a similar device, but only requires one module to chagne any solid surface into a speaker. It sells for 300 bucks, and would be neat to mess with. You could mount it to the inside of the outter door panel and turn your whole door into a speaker, lol (and outside of your car...). Ah, fun stuff.
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU