So I have figured out the Pin-out for this radio and harness.
It turns out that the factory Amplifier in my Altezza powers all of the speakers therefore there does not appear to be a direct connection to any of the speakers on the wiring harness. (The wires may physically be on the loom but not connected to the speakers I haven’t looked any further)
Bearing this and the fact that I think the standard amp and speaker setup is pretty good in mind I decided rather than bypassing the standard amplifier I would use it and connect it to the line out from my aftermarket radio.
Turns out it pretty easy to trigger the amp to turn on and off. My method was to connect the amp trigger output of the new head unit via a 2k resistor to the AMP+ pin on the radio harness. However you could connect this to a switched live. Maybe a resistor isn’t needed but I thought to be on the safe side and not short anything out I would test it with one, it worked so I didn’t remove it.
For the radio to work you also need to supply +12v to the pin that provides power to the radio antenna amplifier (ANT+B). I connected this to the ant + connection on my aftermarket head unit however this could also be connected to a switched live.