It`s called (generically, anyway) a "rear valance panel". Go on the oembimmerparts.com site, and type the last 7 numbers of your VIN into the searchbox, and they will give you breakdown diagrams of your entire car. Then you can order it from them.
I have a missing part on the rear bumper and please tell me what is it called and where can i find it. It screws up whole beauty of my car.