They're called "spring nuts" or "push washers". They look like little washers with teeth on the inside that grip onto the plastic "stubs". They slide over the stub and the teeth bite into it and hold it in place. Part# 64111364257, item# 10 in this diagram:
http://www.realoem.com/bmw/showparts...58&hg=51&fg=15 They come in various sizes and you could find them at your local auto parts store in the "help/misc" isle.