With extra time on my hands and having this problem myself I thought I'd make a How To guide for what appears to be a fairly common issue with the Mk1 Auris.
Thankfully it's very cheap and simple to do. I hope this guide makes it easier for other Auris owners.
There's a previous guide here that includes how to unclip the boot lining:
One more note: On the passenger side make sure you unclip the boot light cable before pulling away the boot liner too far.
Ok, once you have access to the rear of the light cluster there are a few things to undo before the cluster can be removed. I found it easiest to remove in this order.
A - 2 x 10mm nuts
B - Electrical connector for lights
C - 2 x blue plastic clips
I've read that some people struggled with the blue plastic clips, but I found that if you push in the tabs with your fingers (or small pliers as I had to do on one of mine) and push them inwards it takes little effort.
The electrical connector takes a little persuasion. I found wiggling it free from underneath (being very careful around the wiring) with a large flat headed screwdriver worked well.
With all these parts released the light cluster can be pulled straight back. as you can see there's a fair amount of rubbish back there. In many parts the foam rubber gasket was slimy where it was failing to do its job properly. The failed gasket just pulls away from the cluster.
To give the cluster a good clean I took the bulb housing out. The main part just needs the tabs pushing in as in the pic below to disengage the tabs.
The outer bulb just takes a 1/4 turn.
Once these are out it's much easier to give everything a good clean up. Ensure it's all dry before the next bit.
Get the light housing of the bodywork really clean and dry, paying special attention to the OUTER edge of the large hole.
Once clean and dry I went around with clear silicon as neatly as I could.
While this was still wet I carefully replaced the light housing. The long 10mm screws really help lining it all back up, there's also a white plastic lug on the side of the bodywork that helps NOTE: Leave the bulb holder out at this stage!
With the light cluster back on and tightened up, you'll probably see excess silicon appear on the inside edge. Just wipe this away if so.
Then it's just a case of popping the light holders back in and replacing the boot lining.
So far my boot has kept bone dry after being tested with the recent storms 🙂