OK, correct me if I am wrong, but wasn’t that item you found entitled Navigation?

The HK radio when first introduced had an add-on navigation unit (no longer available) that the radio would access through that selection. That option is obsolete and is to be ignored (unless you bought the unit at the time.)

The Road Tech Zumo GPS came shortly thereafter and can interface with the HK radio using the NIM (Part Number 76000024A.)

You said you have an original Garmin GPS, what ever that means.
If it is not a Road Tech Zumo (which was made by Garmin and shared model numbers) it will not interface properly with the NIM (but you MAY be able to get a download from Garmin to correct, but don’t hold your breath.)

Make and model of the GPS are important.
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I have been using my cell and it’s fine in town. But on long road trips where cell signal is scetchy it burns up the battery. The GPS is much better for direction I put in. It doesn't recalculate and send on the interstate when that’s what I want to avoid most of the time.

 

When a Road Tech Zumo 660 or 665 the Harley branded model is mounted either on bars or mount on inner fairing it gets power. If the previous owner put a NIM on the bike there would be a wire plugged in on the front bottom jack on the right side. This is how you get zumo directions out of the 7 pin connector. no other wire needs to be attached to the zumo. The other advantage to the NIM is when directions are given the music volumes drops until directions are complete. Makes it so you don't have to stare at zumo to know when next turn is.

 

If you are talking about a wire plugged into the Aux port on the radio, not necessarily so.
A Road Tech Zumo 660 (and others) interface with the HK radio through the NIM and is wired into the radio harness under the fairing. This will allow voice directions through the radio, lowering the music volume of the radio or CD when necessary.
(The Road Tech Zumo outputs a signal that the NIM needs to do so; the Garmin Zumo does not have that signal. Some have reported Garmin was able to send them an update to include that signal for the Garmin Zumo, but I can not confirm that.)

The Zumo is also capable of storing MP3s and playing them, but not through the NIM. To play them there is a wire from the cradle that can be plugged into the Aux port, then to hear them the radio must be put in Aux mode. It is not needed for the GPS to function. (also, some had their radios modified to place an Aux port on the rear of the radio to keep the front of it clean. So in that case, even if hooked up you wouldn’t see it.

Hope this clears things up a little.
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I think you are correct about the wire in the aux jack. Mine may have been the XM coming in from the 665 xm puck