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.