My 300SD will allow me to unlock the driver's side door from the outside, but the other doors do not automatically unlock as they should. However, if I unlock with the key at the trunk, all car doors unlock properly (so the vacuum system and actuators are working at all doors). So what should I be looking for to fix this? I speculate there is a switch in the driver's side lock that is not activating the unlock "signal" for the other doors.

Sounds like a broken wire at the hinge on the drivers door. This is more common than you would think but it's easy to splice in a new section.

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1983 300SD
200000miles

I may as well agree.....This is my problem, as well.........I need to get around to the fix.
I am also posting for the first time in over a year......Previously, all of my posts were DELETED, because I had not posted for six months......I went and found out that I was "gone".......They let me "back", however all of my previous posts had been nuked.
Sorry for the rambling.
Best wishes always,
spo123