Well Ejaz bhai you can attach MP3 USB SD card kit with your car stereo and play songs on usb or thru SD card.
This kit is called Mp3 USB FM receiver kit..It is very small size with options of Line IN and Line OUT and USB and SD card port..
If you want to do it yourself then you have to attach its line out to the input pins of your car stereo circuit board.
This kit operates on 5v and NOT ON 12v, so you can get a 5v regulator from same market and attach with it to give it 5v supply..
I used it in my cultus and works very very good.