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The Sena is an helmet to helmet intercom system and does not plug into the Harley system nor work as you describe.
Sena makes a variety of headsets and adapters. I have a SR10 adapter , on my Heritage, into which I can plug a handheld CB radio, GPS, MP3, phone or radar detector. All function wirelessly. http://www.revzilla.com/sena-bluetooth-communications
The Sena is an helmet to helmet intercom system and does not plug into the Harley system nor work as you describe.
The Sena SM10 (I'm assuming that was a typo when he said HM10, unless he really meant SMH10) interfaces with the Harley 7-pin connector, in theory giving you wireless intercom, CB, and radio if you have paired bluetooth units in your helmets. That should give you all the same functionality as being tethered with a wire to the helmet.
If not, tell me why. I have SMH-10's in both helmets just for intercom since I'm not on an Ultra, but the SM10 seems to be billed as exactly what OP was looking for.
All are Bluetooth devices and yes they do have a Dongle that plugs into the Harley 7 pin Din plug. The Dongle converts the audio which you would normally hear if you had a wired helmet on to Bluetooth which connects to their Bluetooth helmet system. It only works for listening to what is playing on the radio you can not use the Bluetooth helmet 's microphone to talk on the bikes CB or on the bikes intercom system. You can Bluetooth it to a GPS, smart phone or another Sena helmet interface. Short story it will not give full functionality to the Harley system.
Lots of people make Bluetooth dongles and they all work the same as far as what functionality they give you on a Harley.
Just installed the HD Bomm took longer to remove and reinstall the outer fairning than it did to hook it up. Works great and the install is just as easy as the directions read. Never thought I needed to be able to make or take phone calls while riding, but now that my wife is ill it will be convenient if she needs to get in touch with me when I am on the bike. Best peace of mind upgrade yet and the price is right also. Thanks Jim for a great product.
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