What is it for? If its for a power socket in the boot, then I'd use the fuse box in the dash. There is an allocated fuse for a rear power socket. Just need some auto power cable and repair wire 000 979 225 to connect it to fuse box. This is what I have done. I use it when going camping to power the electric coolbox.
If it is for an Amp etc then I'm not sure, but I'm sure some one else will be able to point you in the right direction.