I ended up removing the kick panel and lower rear quarter panel. But to remove the lower rear quarter panel you need to remove the rear centre panel in the boot and the rear speaker cover, not to mention you need to undo the seat belt anchor bolt. I can't remember if you need to remove the upper rear quarter panel or not. If you do you need to undo the upper anchor bolt for the rear seatbelt.

I was wiring up my whole car so it wasn't a big deal removing all the panels. It's a bit excessive to remove every other panel in your car if you only want/need to remove the door sill panel though.

Then again, there is the grunting and groaning method where you flex the panel and swear a lot, hoping you don't break or scratch the hell out of the panel or any other adjoining panel.

I've learned my lesson, hence the long winded method.