You beat me to it! I'm a newbie too having just got my first Alphy (07 plate, 2.4l) and, short of learning Japanese, I'm also looking to swap out my head unit (NH3T-W56, which I think is a Panasonic unit).
The guy who supplied my Alphy (Steve at SBT Hobbies & Autos, Gosport) has been really helpful and assures me that he can get wiring harnesses to suit Pioneer Head Units, so he may be able to help you / me. What I'm really interested in is whether the existing reversing camera will work with a new Head unit. I somehow doubt it - but once again, any advice gratefully received.