You could use one of these:
Chrysler auxiliary input adapter chry02-aux radio aux 300m pt cruiser caravan mp3 iPod
...which plugs into the connector on your stock stereo where the (dealer option) CD changer would be plugged in. You would plug your phone into it via a mini phone plug to double RCA adapter cable. When in use, the aux input causes the display "Disk 1, Track 1" on the stereo, and can be selected via the "Mode" or "CD Changer" button on the stereo.
I paid $45 for one of these entirely analog devices. Both the phone and stereo volume controls would work at the same time. For more money (at least $150) there are fancier digital adapters that will display the MP3 data on the stereo and allow you to select tracks with the stereo buttons. Some will also recharge your phone.
This was my choice for MP3's on an Android tablet. Years ago, I used a PC laptop.