Interior light stays on sometimes. Why?

[m7m]

The rear interior light (the one over the back seat) of my W210 stays on sometimes. Opening and closing the rear doors make it switch off.
1. My guess is one of the rear door sensors is dirty and needs cleaning. Where is it located exactly?
2. Could it be something else?

 

[Diesel Benz]

The door switch is located next to the door lock "hook". It often isn't about dirt but either broken, misplaced or wear has caused larger door movement and the switch does not close properly in all cases. If the switch looks OK, try putting something soft that extends the tap and makes the switch close better.