HD had such potential to make this all work happily - just that they got it all wrong!
The ideal as I see it, is to have HD's original Bluetooth interface module that connects to your audio system. It works great. All controlled by your left switch controls, your phone book is uploaded automatically, caller ID works, everything. A thousand times better [but discontinued] than trying to use the Zumo's Bluetooth. BUT - if you use a "proper" GPS, where you'd want to have the route instructions interrupt whatever you're listening to, you can't use the NIM with the HD Bluetooth module. HD's Bluetooth module uses the same lines to the HK system as the NIM does, and you can't connect both.
The NIM falls down further in 1) not working with non-RoadTech units and 2) even with them, you can't listen to MP3s off the Zumo without plugging an ugly cable into the Aux - though of course you can put a second Aux socket on the back of the HK Unit.
What HD SHOULD have done was to come up with something like the Cellset, into which you can plug the audio out from the GPS and it interrupts whatever audio source you're listening to. But they didn't.
I've installed a NIM, and as long as you have the later instructions with the wiring correct, it goes in and works easily. This weekend I'll be installing a BOMM on that bike, so the Bluetooth can be used for phone calls through the headset. Not a bad solution at all, but not as good as the HD Bluetooth and the Cellset.
One thing had me stuck a little bit when I installed the NIM - and I can see it's puzzled a lot of others. Bear in mind that it will interrupt the audio BUT YOU WON'T HEAR THE ROUTE INSTRUCTIONS if the speaker switch is in the centre position! You either need the switch all the way back, or you need a headset on!
Jim