Water entering through wheel well grommets is not very likely (unless I misinterpreted your writing).
Water can enter the trunk
1) through the water hoses of the sunroof (the rear hoses of the sunroof go through the trunk along the wheel well until they exit the trunk behind the bumper),
2) at the taillights when the sealing is broken/damaged,
3) at the trunk lid rubber when it is either damaged or after it has been removed previous. Normally there is a sealant like additive inside the trunklid rubber, which can cause the rubber to leak after it has been removed and replaced.
Your rear screen is glued in, so it is very unlikely that water comes in from there (this is more a W201 problem).
with a minor 600+k