The rear vents on this Golf are leaking, which is a very common problem on all makes and models of cars of this age.
The vent acts like a one-way valve with little doors that open when air pressure inside the car pushes them open. These flaps do not have a physical hinge, they are just a flexible piece of plastic, and over time the plastic dries out, becomes less flexible and springy, and so won't close properly. They can also get gumped up with dirt and grit.
The best solution to fix this is to just replace them with new ones, as they are not an expensive part. Unfortunately, they can be a pain to replace on most cars as they need to be done from outside, which means removing the rear bumper.
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