Are you saying the "Push Down/Pull Up" button will not move ?
(As well as the actual locking mechanism refuses to release?)
The driver's door has three parts to the "Lock"
1.The mechanical Lock (And linkages).
2.An Electric Micro-Switch.
(How the "lock" communicates with the Vacuum Pump under the Rear [Right] Seat and the Convenience Control System)
3. A Vacuum Actuator.
(That Locks/Unlocks the door @ Vacuum Pump's command)
These are the Pertinent Sections of S.M.:
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD2/Program/Chassis/72-240.pdf
http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD2/Program/Chassis/72-250.pdf
Unfortunately,another case where all the info needed is scattered all over the S.M. . If at all.
Here's the whole "Shooting Match":
http://www.ps2cho.net/downloads/MB%20CD/W124/Main.html
[I "Think" (so infrequently,it hurts) you've just got a "Mechanical" (Rod) connection loose.]