I second what Fuzzball says. My 84 500SEL doesn't have a vacuum lock on the trunk as far as I know. It is locked by an electric actuator operated by the alarm system under the passenger foot rest. When I first got my car, I knew less about MB'S then I did about women which is nothing. On one of these cars, if you lock the car from the drivers or passengers side, the trunk locks and no number of keys will get it open. My owners manual also says there is a cable under the rear seat but I have never been able to find it. Try unlocking the drivers door with the key and then try the passenger side. May also help to tap on the passenger side foot rest, or toe board or what ever it is called , with something. If it is a bad solder joint in the alarm controler it may help. The above is just food for thought.

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84 500 SEL (307,xxx miles)