OK,  but I don't think there is a feature such as you request for the
back seat lock?  My 1986 190e doesn't seem to have that at least.

Perhaps removing the knobs from the back seat locks will help?  Or
maybe that will just allow for small food items to enter the door?

marty

I misread your post and posted some replies that don't answer it. Try again.

All locks lock but only the driver's door unlocks. Suggest there's a
vacuum problem, most likely in the driver's door lock/vacuum
switch/actuator or a broken or disconnected to the other locks' vacuum
line. A leak elsewhere is also possible.

Since you have this project to do I suggest you bite the bullet and buy
a vacuum tester - about $40 - to trouble shoot this and any subsequent
vacuum problems. A shop will charge $40 just to take their tester off
the shelf much less solve your car's problem.

Woops!  Now that TG points it out, I misread it also...  Certainly the
drivers side key lock should actuate all of the locks...  Except maybe
with the valet key?  Not sure about that, but it seems likely that you
don't want a valet in the trunk?

Probably a vacuum leak I suppose, or perhaps a mechanical tweak in the
drivers door.  I know that on some occasions if I turned the key just
slightly shy of the whole way, my drivers door unlocks without
unlocking the rest,  so that is possible...

Marty