I've never heard of an adaptor allowing control of a generic MP3 player from the stalk controls via the CDDJ protocol. If the only connection between your MP3 player and the head unit is the jack-to-jack aux-in cable, there are no control connections available. You'd need some sort of cobbled CDDJ-USB connector, but a) I have no idea how that would possibly work and b) it would rely on your MP3 player allowing control via USB.
I'd imagine you could do such control with remote-stalk-compatible aftermarket head units with USB mass storage functionality, or if you don't mind the extra screen, you could get a Harmon Kardon Drive and Play like mine. Look for the more expensive Drive+Play 2 if your player is not an iPod, but then you may as well get a USB MS compatible head unit instead.
I'd imagine you could do such control with remote-stalk-compatible aftermarket head units with USB mass storage functionality, or if you don't mind the extra screen, you could get a Harmon Kardon Drive and Play like mine. Look for the more expensive Drive+Play 2 if your player is not an iPod, but then you may as well get a USB MS compatible head unit instead.