Hey guys I don't know if it has been raised on here at all but you can now connect an Alpine 6 disc mp3 stacker to your "E" series factory headunit.
The stacker required is a cha-s634 which is an "AI- net" stacker but has "M-bus" connectivity via an inline adaptor. The adaptor was offered to me for around $40.
So if you are using an Alpine stacker already you can update to mp3 very easily.
Cheers......