The door insulation needed to be replaced as the old was completely dry and had started to fall of. I started by carefully removing the plastic adhesive film so that I could get access to the inside of the door. I removed the remainders of the insulation with a plastic scraper in order not to damage the metal. In the lower part of the picture you can see the remaining old glue which was removed with pure gasoline. I bought some new insulation with self adhesive on the back. The thickness of the insulation should be maximum 2 mm to ensure that it does not interfere with the window.
Here is the finished result and I also inspected the window lifters which were fine.