I don't have a w205, but that liner in the bottom of the hollow looks like it's just a loose push fit with some tabs tucked in.
Where do you park your car? Is it subject to prevailing wind due to being park up against an alley way? Is it on a slope? Is there a gutter pouring on to it?
What is the little black trim piece on the rear RHS? Is that just loose?
On my w212 E-class, the liner is held in place with large plastic push rivets that push through the liner facing rearwards, into the plastic lock/load protection strip that houses the lock catch. If you stick your head into the boot and look reward towards the rear bumper, are there any trim clips to pop out? Of any plastic nuts?
Assuming the car is parked on level ground, common leaks around the rear of saloons are as follows:
- you have some gray/dark plastic trim running from each corner of the rear window, around the outside of the boot opening, down over the lights and towards the boot lock/loading protection strip. These strips are help in place with plastic push rivets. You can carefully pop out these rivets and remove the plastic strips and clean under them. Refitting is simply the reverse of removal. I've done this recently but used a tiny smear of black silicon with the hole that holds the rivet in the metal panel of the bodywork. Through that hole that holds the rivet is the boot, so if water is getting past there it's getting into the boot.
- seals around the tail lights.
- mis-alignment of the boot seal itself, lack of seating of the boot seal on the metal panel lip, or filth on the boot seal or the underside of the boot where it comes into contact with the boot seal. Clean up with dilute All Purpose Cleaner and then treat annually with Gummi Pflege to keep supple. Make sure the seal isn't obviously twisted and make sure it's seated properly. Unless the seal is visually out of whack, I'd guess that for a 5 year old car it should still be in good condition.
- Third brake light that is a physical component bonded into the boot itself (this doesn't apply on your car I don't think, so you can ignore this one).
- the silver trim strip across the rear of the boot lid. Or more accurately, leaks through any fixing holes if it has them. Remove the boot lid lining, and try to refit the trim strip and use silicon on the bolts/holes/seals.
- The same as above, but the fixings for the number plate in the rear of the boot lid.
You need to get the thing dried out with a towel, and then get some dark paper towels of something in there all over the place so you can see where the water is coming from. Check it regularly.
If you can get the boot liner out and can confirm the sides of the boot are dry, it can only be coming from something central, and that leaves the boot seal, the silver trim strip, the number plate, and possible the lock.
Good luck with the hunt.
. I'll get a picture of mine tonight to see if its as bad
