I also believe that their is a threshold. For my Chevy Astro it is around 15 MPH, but it may be only activated in forward. Once the threshold is meet then the ABS will work in reverse.
When backing out of a garage, the minimum threshold is not meet since the car is not traveling fast enough. I do not know the minimum threshold for Mercedes or if the threshold can be activated in reverse.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles