It's purely held on with metal clips. You need to remove the door sill trim, remove the rear seat bottom, fold down the seat back, open the rear window and prise up the window rubber.
then working from the bottom of the door pillar aggressively pull the trim away from the bodywork. There are 4 clips up the door pillar and three along the top. The door pillar is fairly easy but the middle one along the top can be a ****** as you risk tearing the panel if you pull too hard. When refitting remember to engage the little lip into the hole on the wheelarch as you can't do this once the panel is reclipped.
Some of the plastic clip mounts pay come away from the speaker panel. This is normal as they are only held to the panel with hot melt glue, so easily fixed back on. The metal clips themselves may come away as well so ensure you've pulled them all out of the bodywork and refitted them to the panel before refitting.