See also:

SmartKey
0 To remove the SmartKey (shift the transmission to position P) 1 Power supply for some consumers, such as the windshield wipers 2 Ignition (power supply for all consumers) and drive position 3 T ...

Problems with the SmartKey
...

Cleaning the steering wheel and gear or selector lever
  Thoroughly wipe with a damp cloth or use leather care agents that have been recommended and approved by Mercedes- Benz. ...