This is not as easy at it looks. The interface is a complex one.
The best place I found for information is in this forum:
I personally use the USASPEC PA-VOL11 for my HU-801 without nav. It is an iPod adaptor with an Aux-In. As I do not have an ipod, I use it as an Aux-In only device. I know that for HU with nav, it is a bit more complex, so read the forum.
The sound is great.