You switchblade-type keys should also open the trunk manually...only the valet key doesn't.

And unless you've manually-locked the trunk via the key (clockwise turn where key slot is horizontal), you should be able to depress the keyslot with your finger and release the trunk.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car