The locks have become unsynchronized. Mine acted a bit like that some would work some not(i have after market system, but it acted the same when using the key in the door) I just re synched the system and no worries since. Should be in your owners book IIRC.
Gotta love an E34!!
If you want to get junkyard parts remember one thing, if it was wrecked you know it was running/driving. You may not know how well, but it was.
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