The Galaxy was prone to water leaks (even featured on Watchdog).
Traditionally, it was the sunroof drain channels clogging with debris, which would in turn allow the water to collect, and dump itself over the
. This was re-sited in the later versions...
Pollen filter casings can also be damaged/not fitted correctly when changing the filter at service.
Water ingress can be soul-destroying, and a nightmare to fix. I've seen cars with mould all over the inside (at its most extreme degree).
What you could do is to open the sunroof, and block the drain holes with some silicone - if its this water which is ending up in the car - you are cutting off the source.
What's the worst that can happen as a result - its certainly less damaging than permanent saturation.
If that doesn't improve the situation - clear the silicone out and start looking at the bulkhead - there may be a rubber grommet which has perished, or the pollen filter which is the guilty party.
It may be an idea to get someone to shine a really bright torch from inside the car under the dashboard, and for you to look from under the bonnet to see if light is visible - if it is - silicone it up.
Hope these ramblings help in some way.