If you look under the car just in front of the rear wheels you will see the drains, usually covered with a rubber valve cap that allows the water to drain. Best practice is to remove these rubbers to prevent future blocking and rod the drain out using something like a net curtain wire or thickish strimmer wire that wont bend so easily. Rod the drain from underneath, you can pour a jug of water down from the top to ensure they are clear. Well worth doing a couple of times a year especially on the left side as the roof motor lives in there and can get submerged and cause it to fail, best to move the motor/pump to the boot so it can't get wet but that's another job that the brave take on, Alan Number5 on here is the expert for roof and relocation, based in Bristol and can do the job for a very decent price.
See a previous post on this with some photos here
https://z4-forum.com/forum/viewtopic.php?t=126717&hilit=drain+roof+drain