Hi guys - I'm also a newbie to the forum - but with (some) experience of Phatbox cabling to VW / Audi headunits.
I can confirm that the Phatbox IS compatible with the newer VW RCD300 headunit, but requires specific cabling - the std changer cable cannot be used (pre-installed or otherwise). You need to use a) the std. Phatbox to VW changer cable adapter, e.g. 26 pin to 12 pin MOLEX changer plug, b) an older VW changer cable, 12 pin MOLEX to 8 pin mini-iso, and c) an additional adapter at the headunit end, to convert the mini-iso fitting to the newer Quadlock fitting used on recent VW / Audi HU's.
Not sure if this is allowed (?) - but I can supply all cable parts. Contact via pm.
Hope this helps someone, and - GREAT FORUM !!