Indeed the fuse box is located behind the cover in the right cupboard/storage panel.
Itís also listed in the manual of the car.
If you want to run a wire from the live 12V line of that fuse box you need to preferable remove that right side panel.
Not too difficult:
1. Remove black plastic boot panel which runs from left to right (2 outer screws, 2 inner screws and 2 luggage strap torx bolts).
2. Remove right side plastic panel which runs up the boot pillar, hold with 2 screws.
3. Remove right side luggage torx bolt.
4. remove right side 2 plastic clips of the panel next to the spare wheel cover.
5. Remove spare wheel cover by removing at the hinge the plastic clip at each end.
6. Fold right rear seat downwards.
7. Push right side panel out spring clips on the top side.
8. Disconnect right side boot lamp wire.
9. Pull panel to the middle of the car and push it towards the rear seat since the back of the right side panel is mounted around the rear seat lock hook.
I did this now like 5 times (earth connection check, towbar installation, reverse camera installation) but still itís all from memory so donít blame me if I missed something.
Then you have full access to the +12V wire going into the fuse box.
Before you start soldering, disconnect the battery.
I removed the coating of that +12V wire on 1 side, and spliced/soldered an additional wire to it so it.
Make sure you donít damage the strands of the wire and that you properly isolate it.
The rear fuse box is used for several systems which my car does not have, such as the Bose amplifier which has a fuse F61 which is rated at 15A. Since the harness is all the same, if you have all options or not, the main fuse cable is rated for this Bose amplifier and all other user.
Therefore I used this +12V wire to power the electronic modules/lights of the tow bar which is probably max 2A during usage. So there is no risk on overloading the carís original harness.
Again, itís at your own risk.
Hope the explanation is clear enough.