It took me way too long to figure this one out, but finally, the water leak has been found.
It was the roof antenna gasket - or rather, the gasket being absence.
This would cause a very moist interior, electronics would go haywire (doors not locking/unlocking, windows would work sporadic) and mold would develop in the car.
Overall, not very nice. It's very easy to spot if the Polo is moist or not on the inside:
As a temporary fix I put some acid free petroleum jelly around the seals - the same I've used on the gaskets for both rear lights last year.
Before and after the petroleum jelly got applied:
Of course, this is not a long term fix. As soon as the temperature rises, the petroleum jelly starts to runs down from the roof antenna. And with that, the seal will become leaky again.
My older sister wanted to help me with this, and why not, let's do it together :>
Sadly, I did not focus on making pictures that well this time. But the pictures I have will give a solid idea of what happened.
The roof antenna is fixated with a 22mm nut, which is quite hard to reach in a place full of untreated glass fibre - above the headliner.
For this you have to loosen three trim parts, as marked in the picture below.
With some fiddling, the nut got loose and the antenna popped out with no problem.
This mess was what the petroleum jelly left behind:
The watery looking part is also petroleum jelly, which already started to run off.
Next, replacing the o-ring:
My sister made sure the mating surfaces were nice and clean.
And putting everything back together, from the gasket itself I do not have pictures from when the antenna was detached from the car.
This was fairly straightforward, there's only one way it would fit on the roof antenna.
The roof antenna automatically aligned itself with the roof, and the o-ring made a good seal.
Finally, we were done! :>