2017 Mk 3 rear accessory socket.

I run a feed from an ignition controlled feed behind the glovebox to the rear of the car where I have a further auxiliary socket and in-line fuse in a tupperware box in the left hand cubby hole. I use this socket to power my rear facing dash-cam.

How do get the rear auxiliary socket off to attach my earth for my extra auxiliary socket to it?

At the moment it is earthed using the left hand bag hook mounting screws but it looks a bit messy. I can't seem to find a decent earth point in the boot which is easily accessible. I know I should have originally run a twin cable but a single wire is hard enough to conceal behind the door trims etc a twin core would be next to impossible.