I installed a Road Tech 660 and powered it up with the head light wires. Seems to work good. So, now if I want the prompts to come through the radio I need to install the NIM and plug it in.
If you often listen to say the FM radio when you are riding and you want the radio to stop playing the FM station you are listening to when a turn prompt is present then yes you will need the NIM.
Now say you do not listen to FM radio when you are riding but you listen to your own music that you have loaded onto an SD card that you have in your GPS then the answer would be that you do not need the NIM.
The NIM will automatically mute whatever you are listening to (FM, AM, a CD) when a turn prompt is present from the GPS. You will hear the turn prompt and then the NIM will return the radio to the last function you were listening to before the turn prompt (FM, AM, a CD, etc.).
Now say you have a NIM installed and you want to listen to your MP3 files or a phone call from the GPS then you will still need to split the audio output from the GPS and route the audio output to both the NIM and the Auxiliary Input on the radio. The NIM will only interupt the radio for a turn prompt it will not interupt the radio for MP3 or a phone call. If you have the 665 model this would also be true for XM radio via the GPS.
Jim