Ken,

I had a same problem too.
I read through some articles and it could be from the harden weather stripping from hot weather. They are out of shape or came out from tracks. There is a original BMW product called "Gummi Pflege",which will soften the rubber and recondition to original softness according to its description.(dealers do not carry this anymore) I bought it through Internet and I will try this to see it will stop leaking into the door panel.
While I am waiting for this product to arrive, I was looking very very closely at the bottom window weather stripping yesterday afternoon and i found in very surprise that some area of the stripping came out of its track and creates a tiny gap in between a glass and a liner. I just use my finger to realign and it seems Ok at this moment. I will test with the hose this weekend and I will let you know the result.