Since you said that you have had problems opening the trunk unlocked, I think I know what is wrong. I had this problem on my '73 450 SLC. What happened was the latch in the lock had worn to a point where pushing the button would not release the trunk's latch. I got the trunk open (I honestly do not remember how, that happened 20 years ago) and removed the lock mechanism. I drilled a hole in the latch, placed a screw in in the latch, and the lock would now contact and the trunk would open.

Last night I jiggled, wiggled, and pounded on it and finally got it to turn. I think the last post is correct, it's worn to the point where it will only work the latch when it's turned all the way ccw (past the horizontal; it won't open with the lock turned to the "unlock" position, only when turned about 45 degrees past that). Still don't know why the key wouldn't turn, but since it's effectively "locked" all the time I'm not going to turn it vertical again for awhile. I'll try removing the latch and seeing if I can figure out where the wear is.

When you examine the lock - there is a little plastic cog that is part of the locking device - this is what wears out.

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On my '73, there was no plastic anything, so definately look into that. Do not forget to look to see if there is any metal to meal wear.

Now onto the locking mechanism. When you take it apart, lube the inside thouroghly. I find the spray version of "Prolong" to work well for locks, but any good general purpose lubricant may work.

If this does not work, then the jiggling that you had to do to open the trunk may have bent something in the cylinder or disloged a pin in the mechanism. If that is the case, you may have to have it serviced. I can't help you with that since I haven't had to repair a locking mechanism since the '73.