I'm going to add some confusion, but also some useful info. First: I don't have a 590. What I'm adding below is ALL in reference to the 665 unit, and MAY be helpful in relation to 590 but I don't know for certain; YMMV. Here's what I learned.
I recently installed a Zumo 665LM on my 2010 Ultra, adding the dash pad mount. Mine is not the Road Tech, it is the regular Garmin Zumo, so, no dealer POIs and no bar and shield logo. Also, no NIM functionality.
Second, despite what you'll be told by people who assume, there IS a difference in the software in the Road tech version.The software for the Harley unit 'enables' the operation of the NIM. The regular Zumo 665 will NOT work with the NIM. I had to ask a lot of questions of a lot of people and pretty much everyone got it wrong, assuming it would work. I was actually on the phone with a tech at Garmin who ended up getting straightened out by one of their engineers on this point.
I really wanted to do the NIM, NOT because I really needed its function but because I wanted my install to be exactly as capable as my buddy's factory install. I had to give up on the NIM, though.
Third, if you do the NIM with the Road Tech, installed as the factory does it, you lose use of the AUX jack on the HK radio. There IS a work around for this, but it's quite involved, and , again, not the way the factory does it.
Oh, also, the NIM is a separate purchase, not included with the Road Tech.
Fourth, Ultimately, the NIM function is not a biggie to me because I use a Sena BlueTooth headset for everything. I only rarely turn on the bike's speakers, so its not an issue.
I use a Sena SM10 transmitter to send the HK's output to BT and I also BT the Zumo and my iPhone all to the headset.
The headset usage (in my case) eliminates the need for the NIM, anyway.
But what the heck is this NIM?
The function of the Navigation Interface Module is very specific and quite limited: it hooks up to the Road Tech and the radio, including its AUX port. When the Road Tech sends a NAV instruction (Turn By Turn voice), the software in the Road Tech triggers the NIM to 'take over' the radio output so that you'll get your Turn By Turn prompts through the speakers, regardless of which band of the radio you happen to be listening to.
Note that **that's ALL it does**. It won't give you TBT if the radio is off. It won't give you TBT *loudly* if the radio is turned down.
It WON'T send your media or XM through the radio, (using the NIM) you have to switch the HK to AUX for that. The NIM has *nothing* to do with music or media.
The NIM really doesn't possess much magic... if you DON'T have a Road Tech version or DON"T have a NIM, you'd still be able to plug the GPS into the AUX port and hear it that way, but you'd have to chose between listening for the GPS OR listening to the radio.
You could also NOT hook it to the AUX port... just listen to the GPS's own speaker...or pair it with a headset as I do.
Maybe something helpful in there.