The trunk lock on my '87 560SEL seems a little broken. I can unlock it with the key, and doing so will unlock all doors, but I cannot lock the trunk without locking one of the doors (which locks all doors, including the trunk). Obviously this isn't a big deal, but it can be annoying. If I try to lock the trunk from it's lock, the key will barely move, I assume it should go one quarter turn clockwise. Any ideas?

Your lock's acting as designed; it's intended to work the entire central locking system (doors, gascap, trunk).

However, if you turn the key 90 degrees (I believe clockwise)and take the key out, the trunk will stay locked even if you unlock the doors (from the door key). A valet feature to keep the trunk secure as I understand it. The actual valet key won't work on the trunk lock nor the glovebox, so if you've properly locked the trunk it's secure.

To lock the trunk at the trunk, turn 1/8 turn clockwise; that will lock everything. If it requires a lot of pressure, then something in the mechanism needs lubrication but I cannot help on that point

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All the best, Michael
500E
300TE

My 560SEL did similar things with the trunk lock. Try getting a new key made. It may not be actuating the pins in the lock as it should be. Hopefully, you have an original that is not worn down. Good luck!