Hello
the purpose of this post is to help others with the same issue.
Around 3 weeks ago i noticed that i had more than usual fog on the inside of the car. I though it was not so bad and i bought one of those silica bags to absorb ambient moisture. It did not help since I had always so much moisture that the window were completely wet on the inside. We live in Sweden and here it rains 8 out of 12 months....
I opened the trunk and I found the carpets completely wet. The I pull up all carpets and found a lake on the reserve wheel space.
After reading the forum and around 80% of google results on this topic i decided to remove the trims on the tailgate (they were half wet - only the lower half). When removing the trims (carpets) i found that the water was leaking from a cable that was coming down from the side of the tailgate, so i hanged a bucket and left it all night (luckily it rained that night). The day after, i found that there was no water anywhere and all was being collected into the bucket.
When I bought my car Skoda changed the rear windshield under warranty (defogger electric filaments stoped working) , and 4 months ago Skoda changed the valve the spray water to the windshield. With these in mind i wanted to try the water tightness of the rear windshield and drove to a workshop that is specialised in windshields. After explaining the water leakage problem the guy took a pressure air hose with a long noozle and put it right into the side of the tailgate, at the same time he sprayed some soap water all around the tailgate including the third brake light/lamp. When he pulled the trigger and pressure air started to flow it was very easy to see how the air was coming out from the third brake light. I recorded all in a video and send it to Skoda. They changed the lamp and casing for a new one. Problem solved ! no more water in the trunk.
I have already prepare a smoke machine with an old solder I had (found how to make the smoke machine by googling it "smoke machine DIY"). It works fantastic but I did not need to use it.
i hope this help others with the same issue.