1987 5000CS Quattro trunk release

I cannot get the trunk of my 1987 5000CS Quattro open. Neither key will turn the lock cylinder. I have gotten it open in the past without much trouble, but it appears to just not want to turn now.
I am relatively new to this car, so-- how does the central locking interact with the trunk release? I can hear the solenoid near the lock mechanism working when I operate the door lock button but it doesn't seem to affect whether the trunk can be opened without the key, or not.
Of course now it won't open with OR without the key so I am stuck.
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Mark Olson wrote:

Bad form following up my own post, but I figured it out-- the lock cylinder was a mite recalcitrant, which just needed to be forced to operate a few times and I shot some Tri-Flow spray into it.
The central locking was a separate issue, bending the pot metal lock mechanism ever so slightly made it work like new again.
<fingers crossed>
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