Do not understand your problem. You cannot lock/unlock from the outside with key, from the inside by pressing down the plunger, door would not lock no matter what?

As far as I know, all the door mechanism is pretty similar. Just pull a complete assembly from a SEL gas car from a JY. You can key the lock to match or swap the lock but it is a bit fiddly. I would just install keyless remote entry and forget about the key. It is easily done.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.