It's a rather confusing issue...
Now if you download MP3's and burn them to a cd, your stereo has to accept cd-r or cd-rw's, it doesn't matter if that player is not a mp3 stereo...
If your truck stereo won't accept the cd-r's or rw's it wont matter what format you download, the stereo won't accept it....
Does that make sense or have I confused you more...
