I figured out how to replace the middle [3rd] visor on my 1981 300SD.
Very simple after some tinkering.

Just remove the mirror as Don stated, loosen the mirror bracket almost until it is completely off, remove the old visor, place new visor with the male ends aligned into the slots and re-tighten the mirror bracket.

Thanks again Don for the photo of the mirror! Greatly appreciated.

__________________
1981 300SD Turbo Diesel