I had this problem on my '75 280C, which is setup just like the 380SL. As I recall, I drilled a small hole behind the license plate, and inserted a rod so I could move the vacuum operated trunk pushbutton disable link to the right, so it would be in the path of the pushbutton and actuate the unlocking mechanism. The problem, which I fixed, was that there was an interference fit between the mechanism, the disable link, and the pushbutton. The link has to move all the way to the right so pushing the button actually causes the unlock mechanism to move. I just had to "form" things so the vacuum operated link would move easily into place.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C