The door locks on MB are tricky to diagnose because the are part electric and part vacuum, troubleshooting is a pain. I had a similar problem with mine, it turned out to be a bad actuator in the trunk. It stopped the rest of the locks from working. I can tell you of one weak link in the system, I have seen this be a problem on several Mercedes including mine. There is a hose that runs down the right side of the trunk lid from the actuator. It can get a hole in it very easy, that one hole can stop all the locks from working. I just cut mine and got a larger rubber hose to reconnect them. Good luck in finding your weak link. Hope you have the shop manual if you are doing it yourself.
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