https://www.realoem.com/bmw/enUS/showparts?id=UT93-USA-09-2008-E91N-BMW-328i&diagId=54_0354
Here you can see the four drain tubes (item 8 & 9) coming off the sunroof drain rails. These feed into the downtubes that run down your inside A-pillars and D-Pillars. The front drains exit just behind the front wheel wells. The rear drains are behind the rear wheel wells. You will be looking for a plastic/rubber tube with a weird "+"-shaped plug on the end, which is itself a valve on the end of the tube. This is what needs to be removed and cleaned more often than not.
You will likely not be able to access the rear drain tubes without removing the rear glass. This isn't the worst thing to do in the world, but can be a lot of work for little gain. I would only consider removing both glass pieces if you were planning on replacing the sunroof main seal (~$40).