My brother's 2009 Auris was also letting in rain water, even the back seats were getting wet.
After seeing this thread and similar, I determined the leak was coming from the nearside rear corner, and could see it was wet more around the vent than the light cluster. I see some people talking about sealing from the inside, but wasn't sure how that could be done effectively as you can't get to the seals from the inside. So I removed the rear bumper - surprisingly not that hard (just awkward), just two bolts and 8 of those plastic fastener things.
I could see on both sides the foam seal around both vents had disintegrated, and was missing in parts. So I removed the vent assemblies (which can only be done after removing the bumper - they only push out from the car), cleaned up the surfaces, and replaced the foam with silicone sealant. I put some silicone around the nearside light cluster seal too just in case.
I also stuck a dehumidifier in the car for about 12 hours or so, and it collected about a litre of water. Of course now it's stopped raining, so I don't know if my work has been successful.
NB: The bolt thing holding in the spare wheel was totally seized from rust, so be sure to sort that out before you're stuck on the side of the road with a flat tyre and no WD40!