Common issue on all Golfs and other models is the rear pressure vents that are on either side of the car behind the rear bumper. Just done my wife's Mk5 and will do my GTI soon also. Bumper off, vents removed and sponge seal binned while the vent groove is cleaned out......then used a decent silicon to seal and bond vent to car. No more leaks and far less condensation on windscreen too, but interior is still slowly drying out, so can see that only improve with time.
Early/late warning signs it has failed or failing is the amount of condensation on the inside of the windscreen
Image is for example
