I lost mine 2 years ago and havent replaced it yet ($300 at dealer plus painting). I remember driving on the freeway and suddenly ran over something. The next day I realized what it was.
Before I lost it, it popped-off many times when I parked over the curb. You simply push it in and you should hear a snap. It can be tricky the first time.
1987 300D 186Km
1987 300TD 195Km
2001 CLK 430 Convertible 30Km