Sab, Peter is correct. The lights for the Central Locking system will continue to flash with a broken PSE motor.

Below are the id's for the vacuum hose relating to the PSE pump. Remember your car has both a PSE pump and Closure assist pump that operate different components in your car.

ZV Central locking (CL)
RH Backup assist (BA)
SFG Retractable trunk lid grip (RTG)
FKS Rear head restraint retraction (RHR)
P + Pressure connection
P - Vacuum connection

MB Tools and information are available to diagnose problems relating to the PSE pump, however due to the expense, I found it more cost effective to use the "home" mechanic method of finding the broken part.

PSE pump failure is common in your model MB and age.