Ewazix said:
It's worth checking the drains at the front, rubbish can build up in the front scuttle under the screen in the engine bay blocking the water drain-off tubes. Water then makes its way down inside the hollow structure to the sills. The first time I cleaned mine out about ten years ago the front drain was full of leaf compost! See this thread for details https://z4-forum.com/forum/viewtopic.php?f=2&t=40973&p=585592
Bumpy bump.... Drilling holes and removing rear roof drain bungs is unlikely to solve how water is getting in the sills, it flows back in to the chassis sections from the front. Blocked front drains under the bonnet and scuttle area are the most likely cause.