Leak

I have a leak on my 323i. Water is behind the passenger seat when raining. Is this a common problem, if so expensive to repair?

you might want to check this thread:

http://www.automotiveforums.com/vbulletin/showthread.php?t=317138

if E30 is your model, try this:
http://www.bmwe30.net/cgi-bin/datacgi/database.cgi?file=articles&report=view&ID=00000&Section=00