I'm a geek at heart so I get a kick out of things like this, especially for the kids. They think its amazing that I can do this, but in reality its not a big deal.
1) Grab yourself a free MP3 Tag modifier such as MP3Tag v2.48.
2) Open up any MP3 with the program and simply type anything you want into the Album, Album Artist, Comment, & Composer fields.
3) Save.
4) Copy the MP3 to a USB stick
Now when you play the songs on your Camaro, you will see your custom message.