Fellow Member (>400)
Join Date: Nov 2011
Location: Charlottesville, Virginia USA
Thanked 45 Times in 35 Posts
On rereading your post I see it's a leak after a heavy rainstorm so nix those two thoughts.
Have you checked the roof drain attachment? They are a b**** to get at but it might be worth a look. Unlike newer BMWs, the E34 drain attachments are pretty robust, but after 25 years anything is possible. Also, your sunroof cassette could be cracked. So, even though your drain is clear water might be dribbling down the A pillar and finding a path over to your pedal. Pull the A pillar cover and check for dampness.
Remove your cowl trim strip and have a look at your windshield seal.
Instead of pouring water down the drains try fixing a hose on top and let water run over the sunroof for 30 minutes. I would think if the leak is coming from the top you should be able to replicate it with that test.