I DID IT

What a nightmare fitting a new head unit was. I had to get the wiring diagram from the US vehicross and turns out its different to the JDM

Also the addaptor i got to go from Isuzu to the new headunit had a female connector and guess what.......so did the isuzu!!!! so had to make a new connector off the old and new ones!!!! took ages but my advise is follow the wire colours on the wiring diagram and NOT their position number on the connector block as they are wrong!

For the camera feed, there is a yellow RCA video out on the metal box located where the jack is, simply run that to the headunit camera in. For the headunit to show the camera when putting it in reverse you simply connect it to a reverse light feed (which i found under the passenger seat )

So everything still works without the OEM head unit, i was worried the power for the reverse camera and its motorised flap came from th head unit but it doesn't. It all still works fine with the new head unit.