There are now a number of car stereos on the market with a front-panel USB port, into which you can plug a thumb drive with hours and hours of mp3 music loaded on it. I have a boom box, bought at Radio Shack that has this function. One little promo thumb drive that cost me nothing holds some 19 hours of music, and I can load it from music stored on my computer.
You could hide the stereo in the glove compartment, and use small box speakers glommed onto the kick panels with rare-earth magnets. With artfully-concealed sockets for the speaker cords, you could hide any evidence of the stereo for concours purposes, or simply for security when parked.