Yes, the Fatman dates back from the days of the 30 pin dock connector. So you will need an adapter like this one.
Unfortunately Apple no longer make them. You may be able to track one down at a shop where they have old stock. Otherwise you will have to find on second hand.
We've used one in our previous car that had a dock connector to the radio and we've use it with an old Logitech speaker dock.
The only time I've seen it not working was with at a friend's place whose got a Philips stereo contraption with a dock connector. It just wouldn't work with the adapter. After my friend got in touch with Philips it turned out that it was a design error on their part that couldn't be corrected. But if Fatman implemented the dock connector wiring correctly you shouldn't have any problems.