ATV soundbars are a possible alternative for behind the seat mounting. They are almost all designed for the outdoors, meaning waterproof, shockproof and dustproof (resistant is probably more accurate than "proof").
Don't care for the look of this one, but specs are impressive. I suppose it could always be partially covered/camouflaged with carpet.
Boss Reflex BRRF40
40" amplified sound system with Bluetooth®
Includes four 5-1/4" speakers, one 8" sub, Class D amp, and Bluetooth remote
Built-in Class D amp (1000 watts peak, RMS rating not available)
Bluetooth 4.1 remote w/ easy pairing and auto re-connect
EDIT - Found a slightly smaller pic
