What I did
It's not real special, but it works and plays music.
2 6x9 marine speakers. 1 150 amp $50 US cheapo car amplifier, one $50 us dollar MP3 Player, and 1 $50us 520MB sd card. $200 And I had tunes. And a cheap headphone to audio jack cable. I first used a small walled box (metal can w/lid) mounted to the dash for the MP3 players safety, but now I have a mini pelican case.
Yes, Most of my music was downloaded, buy with a chear program, you can convert all you cd's to mp3's.
"Ribbed, for her pleasure"