Ok well I got the multimedia system installed in the car today and it was a quite straightforward process, everything is working as it should so happy days there. I'll go through a step by step on what I have done to install it which might help others when purchasing a similar unit as there is a few little things that might catch you out. Here we go.
Firstly I pulled the dashboard fascia off, to do this just remove the 2 screws above the a/c controls, remove the ash trays and carefully prise it away from the dashboard with a small flat tip screwdriver. Then I removed the old stereo which is held in by 4 phillips head screws. Unplug the old stereo and find the power cable adaptor that is included in the new unit which matches the original cable and plug it in.
I forgot to take a picture but in the new adaptor cable there is a second white connector which is for the rear speakers. There is no plug for it inside the navara dash, I believe Nissan has skimped out on this part of the loom but I am going to see if it can be purchased as I think there is at least some section of the loom going from the front a pillar to the rear speaker cavity. I have no speakers in the back at the moment so didn't worry about it. You could just cut the plug and solder the speaker wires into the correct places if you have rear speakers.