hi,
i would like to know what i have to do to get my g35 2004 radio to play mp3

There is absolutely nothing you can do with the OEM head unit to get it to play MP3's.

Your alternatives are:

1. Replace the factory head unit with a double-DIN aftermarket unit that will play MP3's.
2. Get an iPod and connect it using the adapters that are available (read all threads in this forum section).

has anyone tried replacing the factory 04 unit with a 05 unit?

I dont think so. For it to even look right you would have to replace the entire center console (since 05's were flush)...and I'm not sure if you can do that

05's OEM HU cannot be retorfitted to 03-04 dash's. There are several parts and clips/screws/mounts on the dash that are different between the 03-04 and 05 models.

doc

Sorry to revive this, but can the whole center console be swapped (shifter & stock HU finisher)? Or are the mounting points themselves for the cage and such different?

I'm also interested in this..love to see the upgrade if possible to an 05+ dash.