In my case, the source of water leak on the front-left passenger floor was caused by a failure in German Engineering. I have a E34, 1989. Over the years, I have cleaned and changed all the usually suspects relating to heating & ventilation, sunroof drain, windscreen reseal, vapour barrier, etc, etc. In July 2024, I raised the left wiper arm grill and had a careful look at the seams and panels. I imagined a torrent of water flowing off the windscreen, down the panel and over the 12mm high seam which spans the width of the engine bay along the water channel to exit in the left corner drain hole and the right corner drain hole. It so happens that I spotted a 1mm gap on top of the seam about 50mm from the left corner. This section of seam sits over the speaker panel and is often mistaken for a windscreen leak or a leak from the sunroof drain tube. I used a long handle brush to coat the seam in Lanoguard Grease. After a torrential rainfall where I live in the UK, I raised the carpet. Water leak is finally resolved!
