OK so I have had a chance to do some research on skyline owners and SAU.
Some of the links Google provided lead to invalid topics so not sure whats happening there, maybe a database mismatch.
The posts I was able to read were quite interesting as I was originally only looking to fix up an aftermarket reverse cam to the existing screen by tapping into the existing TV tuner in the LHS of the boot.
What I found interesting is its also possible to put in an AV switch which will allow the running of a UK TV tuner / DVD Player etc.
Some people have also managed to get the system display changed into English language. Sadly im probably about 2 years too late to do this as these posts date back to 2007 and the most recent I read was 2012 from a guy on SAU who was stopping any further development.
Alas my problem still remains because the individuals who where selling the custom cable to connect from old TV tuner to AV switch are no longer doing these.
I cam across a reference to beat-sonic but have not been able to find a beat sonic cable.
I am quite happy to custom make my own cable but I am not great with auto electrics and unless I am using the wrong search keywords, I can not find a post that gives instructions on how to make up the end that plugs in to the aux port (I think 8 Pin but could be wrong) on the old TV Tuner.
I could take all the JDM stuff out but have heard this is connected to other systems in the car, and I am fortunate to have a Bose sound system that is all in good working order.
I will continue searching and update post if I find out any more.