Thanks! Yeah, so same problem then. I have pulled it all out, re-earthed the unit, alternator is giving healthy numbers so really can't work out where the issue is.
Let me know if you find a fix and I'll do the same but I've kind of given up for now. One of these is on the "to get" list though... http://www.aboutsound.co.nz/anycar-units/#itemId=5580ab8ae4b07ca0ea569931
Plugs straight into the back, no messing around - much better than forking out some $600+ for a new fascia, re-wiring the front door speakers and removing the amp under the passenger seat etc etc.