Easiest way to add rear usb charging to T6.1

[dilbu]

I've got a T6.1 kombi and trying to work out the best way to add a usb charging point (ideally ignition cut off) to either the rear seats or rear boot area. Only feed seems to be to the lights which are I assume only on a switched live. I'd love to avoid having to mess about too much, firstly for warranty and secondly as it's quite a new van having to pull too much apart.

Does anyone know of a video or post that outlines the most simple method?

Thanks!