yeah i had a similar thing on thursday only had happen once before, mine leaks on to the top of the rear seats!!! Origanally i took it to the dealers demanding they replace the seals, but they said sometimes the cable for the 3rd brake light traps between the seal causing the leak.

Hope this helps!!

 

That's exactly where ours leaked as well.....
Sod the dealer right off, he'll just pull your pants down with your wallet still in your pocket.
Found the cause of our leaks it & it's a common fault. It's the seal for the high level brake light (not the cable trapping in the tailgate seal as your stealer described). Pull the plastic trim off on the inside of the tailgate & remove the 3 torx fasteners behind it. The high level brake light can then be removed.
Change the seal previously purchased from BMW, fasten it back on & Robert's your Mum's brother.
Kev.

 

thanks will give that a try