The RN-S2 (MFD-2) works by direct plug-in, without an adapter, in the Passat 5.5. (3BG)
It is possible to fit it in by moving the, up to 5, switches from the the sides of the Navi to the bottom below the Climate control. Here is what the lay-out will look like:

To clean it all up I'll make up a wooden face plate from cherry-wood or?
I might modify the bottom of the pit to get it in another 1/4" deeper, depending on what the initial template wood trim will look like.