On older cars, factory had a clamp and pin assembly that you clamp your
floor mat and stick the pin into the mat below that to stop it from sliding.
Alot of time, as these mat age, the foam backing wear out and allows the mat
to slide forward. I think the solution is rip out that foam backing and put
in equal amount of grippy liner that is used in drawer or under the rug.
Or stick a large safety pin into the mat and factory floor mat.
The newer MB has hook and hole so there is no way to make mistake on those..