We have a nice Sony 12VDC car stereo/CD, a 120VAC Bluray DVD, a sound bar and subwoofer. While it's not "surround sound", it does a really nice job with various different settings for both music, movies, sports, tv. etc. And when you turn on the speakers in the garage too it's pretty clost to ture surround sound.
Only thing it needs is an amp, which they stopped putting in this year, although my neighbors might appreciate that at times.
I'd go with a good aftermarket car radio, which will be 12VDC. Then there's lots of small surround sound systems out there, although finding one that is 12V compatible might be your limiting factor.
__________________
2014 Raptor RP300MP w/ Rear Patio Party Deck, Folding Side Ladder, 6 Point Level Up, Carlisle Radial Trail HDs
2004 2500HD D/A CC SB - TTT Mirrors, Prodigy BC, 18K Pullrite Superglide, NEW Fuel Injectors 11/2015 (ouch)