1989 260-E front courtesy lamp assembly
 

Could you tell me how can I open the front courtesy lamp assembly?

The front courtesy lamp has metal clips at one side (I think it is the rear side...not windscreen side) and the lamp needs to be pushed in the direction of the clips in order to get the opposite site out as first.
Once the light is out you can take it apart completely (if that is what you want) by depressing a couple of plastic clips.

greetingz,