Just to close this out, I finally bit the bullet and pulled the trim on the A6 C7 Avant. To help others, I have listed my steps below:
1. There are four Torx 25 screws: 2 under the warning triangle cover (remove warning triange!) and 1 under each of the tailgate panels giving access to the tailgate lights (just slide them off).
2. Now for the moment of faith, Don't be worried about using a bit of grunt to lever the larger bottom cover of. Plastic tools are best and there are 14 individual clips (see image beow). Once you have popped a couple, get your fingers in and give them a good yank - they will pop. Maybe a good idea to have a couple of spare clips - they are a diamond shape with an open bottom with a side flange on either side to slip into the retainer - basically a bent piece of metal.
3. Once it starts coming loose you need to be careful not to let it drop - there are wires going to the boot open/close switch (model dependent) and to the small interior light interior light on the bottom of the tailgate. If you lower the trim when it is loose, you will be able to disconnect them quite easily.
4. The trim surrounding the tailgate window coms off in the same way. The upper back is quite difficult to access, but persistence pays off. It can be a bit difficult to lift out when unclipeed - it's a bit tight.
5. The hinge covers just clip off.
6. Install your rear dash cam wiring. You may have to to file a semi-circuar hole into the plastic trim to allow the dash cam cable to pass through.
7. Checlk all the trim for missing clips and some small rubbers that fit over some holders. Look on the floor and your car boot or any that dropped off on removal. Check clip holes on the car to see if any clips stayed with the car rather than the trim. Bend and reshape (or replace) any distorted clips (quite easily done)
8. Reassembly is the reverse process, but the window trim is difficult to get in the right position and it has to go over wiring etc to get to the windscreen and it is a tight fit. Sometimes when you feel you are near just give it a bang to see if it all goes in. Again persistence (and another pair of hands} work.
9. Retrofitting the larger bottom panel is much easier - especially if you slide the trim into the slot around teh lock as a starter/locator. Don't forget to to push the two wires through before fitting! Most difficult bit is where bottom trim meets window trim. Again, some judicious banging to get clips to align works
10. Once all clipped in place, replace screws and pastic covers. Some more banging to get hole alignment OK may be needed.
11. All done. Test everything.
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