one of those cheap 5.1 (aka 6 channel) sound cards off ebay for a few bucks.
made in china, cmedia chipset etc
all the same, all very stable, all average sound quality, some ppl will suggest spending $1000 etc but hell its a car not a place to /enjoy/ music, $5 for the soundcard $995 left for that cinema projector......
PS I run a pci soundcard, a cheap cmedia one, I use its 4 outs, FL, FR, RL,RR into a 4 channel amp, and also connect the FR and FL to my sub amp. works a treat.