I guess it is the same as on the X3. They are located in front of both the front and rear tires. http://www.dragtimes.com/images/18595-2009-BMW-X3.jpg

I replaced one when I bought my used car. A month later I hit a bad bump on unpaved road with three passengers. I almost bottomed out the suspension and broke both deflectors in the back. Another $25 and I guess the will break easily if driving through deep snow. We'll see this winter