Well you can pull all of the other door panels besides drivers , plug off vacuum lines (make sure to superglue plugs to the vacuum lines cause of pressure) and leave the trunk and gas flap actuator connected and this will do what you want. You will have to manually lock the other doors though. I have installed a sylenoid a couple years ago that locks doors automatically when vehicle put in gear . If you can find a good Mercedes shop in the area you can have this done . Make sure they are qualified before they tamper with the system.
