Thanks to @weyland's excellent instructions, and tips from a few other Brisky members in various threads, I fitted my (long overdue) reversing camera today.
I have some observations based on my experience today that may help people fitting their camera.
1) The media unit has plenty of room behind it. Also it can be pulled quite far out with the cables still attached. However I noticed my unit had cables which were trapped behind something initially, which made getting the media unit to move more than about an inch very difficult.
As Weyland suggested, getting down into the footwell and looking up with a torch allowed me to see what the problem was, and move the cables free to allow the media unit to be almost completely pulled out.
2) I tested the camera in the car. After connecting the main connector, I simply rested the camera on top of the media unit and checked to see if it worked. That way ensured I found out if I had what seems to be the most common fault, poor connection, at the earliest opportunity.
When refitting the media unit, be sure to position the camera wire correctly to the side, as it can get trapped behind the media unit. Ditto the cables out of the new inline double socket. It's quite fiddly so expect it to need some care and patience. And a torch!
Note the connector has a lever-lock action to secure it in place. The lever is at the bottom of the connector, so it's not hard to lift the lever to remove the socket it from the media unit. I found that with the lever lifted, the socket came out fairly easily, with little force required.
I plugged the new socket in very carefully, and pushed it right in, using the lever-lock purely as a lock once connected, not as an aid to pulling the connector into place.
3) I went the Weyland/OEM up and over route, and it was fairly straightforward. I started at the head unit, and went towards the back of the car.
Arguably, this is the "wrong" way to do it, as the connector for going this way is larger than the other end. Something that was a particular issue when I got to the rubber connection between the hatch and the car body.
The tip to use net curtain wire as a guide to pull the cable through was invaluable, but I also used a dowel as a guide for the rear hatch hole to the C-pillar and I think around the B-pillar too. . I attached the cable to the pull wire/dowel with self-amalgamating tape, because it doesn't leave a sticky residue. I also found a "One step back, several steps forward" approach worked best, rather than simply pulling the cable through. I'd gently pull the cable back, to ensure it was moving freely, then pull it through with the curtain wire until I felt resistance. Then repeat the back/forward action. It worked pretty well.
4) Pushing an end clip on the boot handle as well as pushing out from behind (a two handed operation) worked a treat, but it's fiddly and awkward to do. Don't let the pictures fool you, there are a load of obstructions in your way and it's a pain.
Be patient and use careful force on the end clip, and you'll be fine. The electrical connector into the boot handle switch has a little lock that you need to release to remove the connector.
When refitting, slot the inner edge of the connector above the number plate for an easy refit.
5) Removing the rear trim from the hatch is straightforward enough, but it requires some confident force to remove those clips and it's a real PITA putting it back. A 2nd pair of hands is very useful. I've uploaded some pics so you can get an idea of where to expect the clips. Note there are four clips in the bottom (furry) half of the trim which are at a different angle. (Either side of the wiper motor bulge.) Note use of picnic mat to prevent damage/soiling to rear trim.
Also the furry trim slots into the boot lock, in case you were wondering. (See pic.)
6) The dowel was very useful for finding the route from the rear hatch hole to the front of the C-pillar.
7) A particularly tricky part of the job I found, was feeding the socket through the rubber hose between the rear of the car and the hatch. It's a snug fit. Also the hose has hard plastic clips in it, it's not all rubber.
Edit: I forgot to say, I used aerosol silicone spray as a lubricant to both help the cable through, and ensure the rubber seals made good watertight contact when refitted.
I'd advise against using oil on rubber as it does not do it any good in the long term.
https://www.amazon.co.uk/WD40-Performance-Silicone-Lubricant-250ml/dp/B00KPUBRJ8
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Allow about 4 hours, including time for struggling with tricky bits, taking pictures, and generally not rushing and damaging anything.
By the time I was almost done, I was pretty fed up with the whole business, and was replacing the uncooperative rear trim in a pretty foul mood, so I had to try and chill out a bit to ensure I didn't rush putting it back and damage something.
All in all, it is fairly simple to do, but being simple doesn't make it easy.
Good luck!