A door panel or other interior trim could do double duty, opening up in-car real estate for something else where a speaker sits today.
This is fine for low frequencies. In fact, I use tactile transducers in my Home Theater.
Having heard "full range" speakers using transducers made by Dayton. The least impressive part of the speakers were high frequencies. It's only a matter of time before they get that right...
http://www.daytonaudio.com/index.php/catalogsearch/result/?q=exciter
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