The Navigation Interface Module (NIM) is a new product from HD. The purpose is to allow you to listen to any audio input from your radio and when the GPS unit "speaks" it will quiet the radio and play the GPS audio through the speakers or headsets.
I have XM in my radio but there are instructions if you will be using the XM in the GPS as to how to configure it. All this is mounted inside the fairing for a nice clean install.
One other thing to consider is that I sent my radio off to IRON CROSS audio and they put an additional AUX port on the back of my radio for just such an instance. They also installed a nice iPOD connector with about 50 inches of cable to route to my iPod mount (chrome Rider's Claw). This connector takes the uncompressed audio out of the iPod for a better sound and also charges the iPod while riding. I got them to change the screen color to a blueish white, much better than the red, they can do Orange, Green, and Blue too. While they had it, they brought it up to date with the latest mods and did some additional waterproofing to it too. Great bunch of guys to work with, and fast!
Hope this helps!
Brian