Tom, you will have to pull the locking mechanism out and there is the lever that the key turns that moves the lock up and un lockin lever. Mine was bent just enough that when the key was turned it missed the lever, all the other doors locked but not the driver side.
You may have some other problem but that was mine.
Have fun.
Den

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Den
1993 MB 300D 2.5
2001 VW Jetta TDI 5 spd.
2001 new bug TDI 5 spd.
Motorcycles