The Iron Cross option is input only (just FYI). So it would work as an input source from a Garmin, phone, or other device. Would not play back to headphone, helmet speakers or other receivers though.
I'm not sure I understand what you mean when you say it would not play back to headphone or helmet speakers. If it works as an input source, where does it go from there?
I spoke to Tony at Iron Cross Audio at length today. He told me there are two options. First, and best option is a bluetooth interface module similar to the xm and cb radio modules. You install it then you toggle the mode button, i.e., am/fm/xm, cb, weather, aux etc. So a new mode would appear that is for the bluetooth interface. If it is paired to a Zumo or phone, you can select the music and stream it into your wired headphone speakers (that are attached with a cord to your wired harley jack), right from the left handlebar switches and the songs appear in the radio display (and on the device that is paired to the interface). The other option is a bluetooth receiver built into the radio and you still stream the music from your zumo or phone into your wired headphone speakers but it works through the aux mode and you cannot control what you are listening to from handlebar switches. You have to choose the songs from the zumo or phone device.