Depending on what format you have recorded the music on the SD-card, there are many MP3 players that have SD-card reader built in.
Or if you have a mobile phone like mine: Nokia N95, you have both SD card built in, and a stereo jack that you could plug into the AUX input on the stock stereo system. Then there is only a question of finding a program that you can install on the phone that play that format that you have recorded.
But I’m going to change my stereo next year to a Pioneer AVIC-Z2 system and there I have a harddrive that I can download music/audio to.