A friend made horns which started under the seat and flared along the floor, exiting around the firewall. This was not some hobbyist fooling around, this was at a several billion dollar automotive company. UNBELIEVABLE bass from little 5" drivers. Horns basically transform the impedance of free air down to something the driver can "grab" better, therefore much more efficiency. A bandpass &/or arrays of multiple woofers get somewhat of that effect, but not as effectively (pun intended )
Caveat: The folks at DIYAudio.com can tell you about designing the horns, but integrating into the vehicle is completely uncharted territory. You might get better opinions if you tell us about your vehicle and how much space you are willing to give up in what part of it...