I wonder if this is limited to S models or do all 4 door MBs have this switch that disables the inside door handle on a rear door.

 

Another:

[IMG]local://upfiles/7339/62FEF6E4F4DB4482B6C215850B48EA9B.jpg[/IMG]

 

That is a child lock for rear doors which is found in all cars now.
MB had it since late 60s or early 70s.
Children sitting in the rear seat cannot open the door from inside. You can only open the doors from the outside if you enggage this lock.
That is why the police can simply push the suspects into the rear seat of their car while they ride in the front.
Shan.