My '85 500SEL trunk won't unlock with a dead battery either. Doesn't matter which, how or where you use the keys. Might be a fluke with the 1st gen models. Probably a problem with a leaking vacuum system somewhere.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now