Well, not a Nav IV but I just had a Garmin Zumo 665 (the Nav IV's twin) installed on an '09 LT with the IBM Voice II intercom and J&M helmet headsets. I haven't seen what the wiring harness on the Nav IV looks like, but the Zumo 665 was a relatively straightforward install. It isn't 100% "right" yet, but all I have to do to hear the GPS prompts (and MP3 music, etc.) is switch the LT's mode to "aux".
Where I want this to end up is: I hear the GPS prompts while listening to the factory radio, CD player, GPS MP3, or BMW CB radio without changing modes to aux. I think what's left to be done is connect some audio-out cables that the service tech left dangling beneath the radio.
Where I want this to end up is: I hear the GPS prompts while listening to the factory radio, CD player, GPS MP3, or BMW CB radio without changing modes to aux. I think what's left to be done is connect some audio-out cables that the service tech left dangling beneath the radio.
so I had to chop and solder extra cable to hard wire to the battery !! 
