hey how are you
1- you need to check the door lock actuator & replace if defective , did the problem stayed after replacing the remote battery .
2- the BCM ( body control module ) The All-Activity-Module (AAM) (N10) is located on the left side of the engine compartment. It is plugged into the fuse and relay module (F1) from underneath.it controls all door lock activity( i sent you location
so the problem has toi be one of the 2, you start with the door lock actuator .
if you have acess to A Star mercedes diagnsotic scanner ,if you scan the BCM module we can be more accurate
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ASE Certfied Technician