I do not have the CDXM30, however I do have the H910UI with a wired remote. If the CDX uses the same wired remotes as mine, you will need a Y adapter. Something like this (center piece in this kit); RM X55M Wired Marine Remote Control | eBay
However I am not sure what your head unit requires.
Why would you want to "Y" in the 3.5mm ?
Sorry about my my earlier conniption fits :face palm:
Read this thread - Sony Cdx-M60Ui - Stereo Info & How-To - TheMalibuCrew.com Keep in mind this is for my head unit, however I do have the 3.5 connection as well as the DIN for the RMX60M because of the readout- Read philwsailz thread
You're warmer down there so PLEASE let me know if this works for you (see below) - I have another 4 months before I see my boat.