some more information here. Please message me if I can help anyone with further information...
the wiring diagrams I used are here:
http://www.tehnomagazin.com/Auto-radio-car-connector/PORSCHE-Car-Radio-Wiring-Connector.htm
On mine I only had plugs A and C, plus the orange Fibre optic cables.
Sound:
The connect2 convertor turns the Fibre into normal RCA plugs so you can attach to your new headunit.
Power Switched:
The convertor attempts to use Plug C pin1 and 2 to control the ignition on. This didn't work on mine so they suggested I simply ignore it - which I did and routed the power from a switched power source in the fuse box.
Power Constant:
Plug A contains the constant power and antenna power (pin 5) which you simply connect to the antenna power of your new head unit and plug into the convertor
Antenna:
I used a convertor so I could convert the Porsche antenna into the standard Din of the new headhunt. this is just plug and play (ignore the blue powered from the antenna convertor as this is not needed due to the power for the antenna coming from the PlugA pin 5 (see above)
https://www.renntech.org/forums/topic/35903-pcm-antenna/
GPS:
My Head unit came with a GPS antenna (Small square one) this looked the same as the porsche one that is mounted under the alarm light in the centre top of the dash. I did buy a convertor for this but in the end decided to simply swap out the Porsche one for the new Kenwood one, which was easy to do.
]https://www.renntech.org/forums/tutorials/article/20-pcm-2-installation/
Half way down the page on the link above it shows you how to swap them over.
DAB:
I bought a small thin GPS antenna from here:
http://www.dabonwheels.co.uk/Kinetic_MagnetwiG__DMA-1004_DAB_car_aerial.html
I routed it to under the cowling on the right hand side by the pollen filter (as you stand in front of the car looking at the windscreen). I wedged it into the diversity antenna that may or may not help with the reception - it has been fine up to now.
Double Din cage:
I bought this kit again from Connects2. Use the part number on my original post as connects2 tried to give me the part number for the 997!.
It fits well, but I took advice from another forum and added some screws on the left and right side to hold it tight, I didn't think the bend over clips would secure it.
Headunit:
I am very pleased with the headunit. The sound quality is great as is CarPlay. you can customise the colours of the keys and the background to match your car.
Other:
Don't forget to connect the handbrake cable from the headunit to the negative power to trick the unit that the handbrake is on (loads of details about this on the web)
Connect the dimmer cable from the head unit to the cigarette lighter illumination cable to dim the unit when the lights turn on
Connect the Antenna power cable from the head unit to the cars antenna cable