New to your channel and Disco ownership, so I will now watch you avidly. I wonder if the vehicle has been flooded? All that ‘dust’ in the tailgate reminds me of dried muddy water…
I have had problems with the switch that opens the top door, caused by pressure washer detergent ingress into the switch causing it to fail. Fitting a simple press type switch into the plastic panel (on the back outside of the bottom door) is a very cheap and effective fix. You must clear mastic the top of the plastic panel to stop the problem repeating itself.
You should have replaced the latch cable at the same time, because they do break very frequently and when they break it's a pig of a job to open the tailgate to replace it.