The locks on Mercedes are a pain to troubleshoot since they are both vacuum and electrical. I had the same symptoms as you did. This is what was wrong with mine. The trunk actuator was broken. I just replaced it with a good one. Also the hose that runs through the trunk lid to the trunk actuator had a splice. This is a common thing, I have seen it on several Mercedes. The hose runs through the right side of the trunk lid and can apparently get spliced. I just cut the line and put a rubber hose over it to reconnect it. Since then my locks have worked fine. Who would have guessed the trunk actuator was bad when I couldn't open the locks from the drivers door?
MBCA, PCA, BMWCCA & SCCA* member
86 190 2.3-16
83 944 ITS Racer