Do you mean that you don't own an MP3 player you can hook up to the stereo, or that your stereo does not play MP3's?The next thing I noticed was that there wasn't a good place to put CD's. I don't have an MP3 player (believe it or not). I found out that the ledge behind the driver's seat can hold 3 CD's, each lying flat. I bought a rubber material at a dollar store that lies flat on that ledge. With that in place, the CD's don't slide around.
Does the CD player in your car play MP3's off of a CD? You should be able to copy the CD's you have to your computer, and then copy those MP3's to a writable CD and then put that in your CD player. (It's what I did, rather than hook up my iphone each time I get in/out of the car)