Sorry for a late reply!
I haven't removed the rear lights from my car, but have read it is quite straightforward.
Once you have the inner light out, you need to undo the reflector housing / bulb holder (Philips screwdriver needed), and you should end up with the following;
In the pic above you can see the plastic lugs of what is the plastic red fog light / clear reversing lens - you just need to carefully press in the lugs one at a time and then you can remove the filter;
Hope this helps!