I had this problem on my 300se once. If the key turns and the button goes in then I think it is the latch to the side of the key.
Turn the key and push on the trunk, or have someone lock and unlock the doors while you push down on the back of the trunk just to the left of the keyhole.
Thats what worked for me anyway. Then I cleaned up the latch and it worked fine from then on.

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“Fundamentally, sustainable development is a notion of discipline. It means humanity must ensure that meeting present needs does not compromise the ability of future generations to meet their needs.”--- Gro Harlem Brundtland


1989 300se 250,000
1983 BMW 528e 100,000
1999 Olds Alero 80,000

These pretzels are making me thirsty.