http://w124-zone.com/downloads/MB%20CD/W124/w124CD2/Program/Chassis/46-8110.pdf
See if a screwdriver inserted behind the broken key (From the passenger cabin)will allow you to turn the "Tumbler" to Position #1.Then just follow the directions in the manual link above.
IF your "Tumbler" with the broken key will not allow movement Try clamping the
screwdriver handle in a Vibrating Sander ,then attempt to turn "Tumbler".