Same with mine a few weeks ago.
Nothing sitting in the box to prevent the mechanism from working, however.
I pulled gently with increasing pressure/force...she finally popped open w/o damage.
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Mine had one of the two internal releasing arms busted which didn't allow full excursion on one of the two spring-loaded gizmos.
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'86 Ford F150 locking mechanism is identical.
I swapped out the internal part I needed.
If you take it apart and find that one of the arms is broken, PM your name & address and I'll send you the one good arm and other internals I have left just in case the opposite internal releasing arm is broken on yours.
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Be mindful if you disassemble - the spring may fly across the room.
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Good luck.
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ps If you damage the receiver part of the lock (that which is screwed into your box) the '81 > '86 F150 has the identical receiver.
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