If you want to use the 660 for mp3, the only solution is to get a 3.5mm audio cable and plug that into the Y connector that comes with the NIM. Run that cable from inside the fairing to the outside. Then ... if you want to listen to iPod via aux jack ... plu in the iPod (the new 3.5mm cable from the 660 is left unplugged).
When you want to listen to mp3 (or other) audio from the 660, unplug the iPod from the aux on the front of the radio and plug in the 660 cable that you ran from inside the fairing. The iPod cable is then left unplugged. Certainly not ideal but unless you want a direct iPod interface solution (HDPDPLUS or the Moco) - it's the only way I know of to get mp3 (or other) audio from a 660 AND use the aux jack for iPod. One or the other, but as Jim said - I would not recommend trying to use both sources, via a Y cable, to the front of the radio at the same time.