Every time it would rain, I'd find water soaked all through the rear seat & all over the floor. It was especially bad on the driver's side but after a few weeks the passenger side was getting soaked too. The water would pool at the back of the rear seat & in the footwells.

Automaster BMW determined the plastic seal on the inside of the of the driver's door needed to be replaced. It wasn't expensive, but didn't fix the problem at all -- the next day we got a lot of rain, I accelerated & heard a waterfall behind the back seat.

I got mad, took out the rear seats, & noticed the water was leaking down from the top of the back rest. Turns out the drains for the convertible top on each side were plugged -- I park on a slight downhill & after a few inches of water pooled over the plugged drains (which are near the hinge points for the top -- see photos), the water just pours over the lip & spills down behind the rear seat.

I used a small wire to unplug the drains, just jammed it up & down a few times to clear the plug & let the siphon (made a sucking sound) do the rest. Haven't had any water problems since.