If you want to listen to the GPS via the Auxiliary Input Port on the face of the radio then you really do not need the NIM. Just run a 3.5mm Stereo Male to 3.5mm Stereo Male cable from the Audio Output of the GPS to the Auxiliary Input Port on the radio. A 3 foot cable will do just fine and you can find them at Radio Shack.
The NIM is only good for Turn Prompts and will automatically interupt other modes of the radio for a turn prompt and then return the radio to the last mode. For instance, if you are listening to the FM radio and a turn prompt is present the NIM will mute the FM radio, let you hear the turn prompt and then return the radio to the FM station.
If you want to listen to a phone call or MP3 (XM if you have the 665 model of the GPS) then you need to do this via the Auxiliary Input. If you want to be able to talk on the phone via the GPS phone interface then you will need an H-D Bomm (
www.hdbomm.com).
If you have other questions about this you can always give me a call if you like @ 302-670-6368
Jim