I'm going to give you a few facts about my 84 500SEL concerning the trunk lock.
My car had an alarm system which had been disabled except for the trunk lock. The trunk lock is an electric solonoid (sp) that was operated by the alarm system. When it was in the lock position, nothing would open the trunk. When in the unlocked position, then the key worked as normal and the trunk could be locked or unlocked with the key. However when it was in the locked position the only thing that would move it to the unlocked position was a key in the drivers or passengers door. If the battery was dead and the trunk locked, then forget about getting in to the trunk. My car also has the reclining rear seat and I have never been able to find any evergency cable. Since the vacuum pump was missing and a replacement was not available at a resonable price because the car also has the orthopedic seats which share the pump with the door locks, I converted the door locks to electric and used the trunk lock also, so if the doors are locked, so is the trunk.
Check under the sub pannel in the right front floor board and see if there is a small box with i believe 1 plug with 10 or 12 wires. That would be the alarm system, and is the unit that handeles the trunk lock.

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84 500 SEL (307,xxx miles)