Haha,
Just did exactly the same thing - changing out door locks on driver side of my '84 300sd- all back together but can't lock door. I do a search and land on this question, and - oh, yeah, and duh! Can't lock the open door. I go back outside to check and yes, the closed door locks perfectly! Thanks again, even when I should know better.

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles