Eric, some of the best these days are PC surround sound systems which are much smaller comapred to boxes that dont have acostic wol. They cost about 7K to 10K but run on 240volts. You could use an 12 volts 375watt DC to AC invertor to power these ( pls ensure fitting is done by a professional, AC and DC ground are different for safety ) to. If your audio system has LINE OUT, the speakers will work fine.
interesting topic, i've been wondering how i can get my hands on a a good woofer for a station wagon and not those big blanket covered boxes i see people fixing.....any idea where prices are reasonable?