If I recall correctly, there are three lock functions in the trunk latch.
#1. Mechanical key lock.
#2. Vacuum lock actuator.
#3. Electric dead bolt totally controlled by the alarm module.

If the battery goes dead and/or alarm module fails or is removed, the dead bolt locks closed, and the key or vacuum actuator will not open the trunk.

Here is further data.

Trunk lock problem-W126 300SD
http://www.peachparts.com/shopforum/showthread.php?p=879700

trunk won't open
http://www.peachparts.com/shopforum/showthread.php?p=512820

82 300SD Trunk Won't Open
http://www.peachparts.com/shopforum/showthread.php?t=15438



Have a great day.

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1985 300SD
1983 300D

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