The GPS unit I have screws onto rear shelf from the top, meaning the actual cover has to come out to get to it, or at least unclipped enough to allot the front bit behind centre headrest to be lifted high enough to get a screwdriver in, you can't access it from the boot, has to be from inside the car.
Excuse the poor pictures, would appear my phone camera is useless and on the way out, and now I've found proper camera I can't find the GPS unit!
Mind plug does look very similar to the Nokia handsfree kit for the arieal.
If and when I find GPS out welcome to try that and Nokia kit to check fittings
