On a 206 you need to remove the rear seats. The seat cushions just unclip after you flip them up/forward, backrests are bolted in with a couple of bolts at the bottom corners.
Lower seat belt anchor rail needs to be unbolted.
Pull off the door seal from the flange in the lower rear corner.
Same for the seal on the rear side window - don't recall if I had to remove the glass or not, but that just held on the latch and hinges by a screw on each.
IIRC also undo the shelf/ledge thing on the side that the rear shelf rests on.
Now you can pull the panel.
Run wire from HU to the location, following the sills if wiring isn't already present (or you want to replace it with better/fatter) .