.../...it is surely the switch that is stuck (shown in the photo). Can it be repaired ?.../...
Try Alfred's method of removing the pin in order to disassemble, remove the "shell" or "cap" at the end of the stalk.
The halves of the black parts attached to the chrome shaft are permanently joined together.
If you feel adventurous, and you need to do so on your stalk, you can split them along the seam, to aid a repair. The housing (halves) must then be glued together afterwards.
Note that there is an early and a late version of the Pagoda wiper multifunction stalk. The change came about in the summer of 1967 (mid 250SL production)