I have wondered why the screen informs us that the side marker lights will be unavailable during updates. I received a big clue when I inadvertently kept a trailer connected during my recent update to version 2.1.6A.

After accepting the update and locking the vehicle, the Defender went dark, as normal ... but, it was night, so I noticed that the little yellow lights in the rear view mirrors which are lit when a trailer is attached (and the vehicle is on) continued to stay lit. Then a few minutes into the process ALL my trailer lights went full on and stayed on -- until I disconnected the trailer from the vehicle to conserve the battery.

After the required 40 minutes, all worked as normal and the system page confirmed the update had occurred successfully.

What I suppose is happening is that all the switch logic to the side marker lights (as well the sensor's to all the ECUs) are being disconnected during a software update. Then the side marker wiring looms (which run all over the vehicle) are being used to provide continous power to all the ECUs -- so they may each receive the logic updates which impact them, as necessary. Following this approach eliminates the need to have dedicated wiring for the purpose of powering a software update.

I guess we are being told about the side marker lights so we do not execute the updates on the side of a busy road -- where a dark automobile undergoing a software update may be hit by another vehicle.

Enjoy !!