Had the same issue with my 69 wagon that had been sitting for 30 + years. If you have a harbor freight store, go and purchase their blue colored plastic trim removal kit/
https://www.harborfreight.com/trim-...9RoGZuddYwDlINA_F7tG6UzG-EJiE3UxoCvpoQAvD_BwE
The door handles are secured with allan wrench type screws. There are two large phillips screws under the armrest that need to be removed. The door pull is usually a secured with a hex bolt. Check under the metal ash tray for any hidden screws.
The panels are secured to the door with wire spring type clips. Carefully use the trim tool to get between the fiber door panel backing board the the metal frame of the door and pry them loose from the holes in the door.
Once the are all loose at the bottom. with the window rolled down so the glass is not contacting the black whiskers on the top of the door panel, lift the panel upward, it is secured in place with two tent stake like clips that position the panel in the top of edge of the window frame. Remember to un-thread - remove the lock button ; )
Once you peel back the plastic, the door lock mechanism is located at the right hand side. Put some old rags - paper towel down and clean it with your choice of cleaner. Let it sit for awhile and let the cleaner work its way in. Then start working the mechanism - re-spraying cleaner until it works. Lubricate and you are done.
These rear locks are not as smooth easy to work as the front door locks - force wise.
Grab some double sides tape to help keep the plastic moister shield material in place when re-assembling the door.
This would be a good time to clean the window track of old grease with a tooth brush and re-lube with white lithium grease.
Take you time, this is all mechanical stuff, there is not too much that you can do that would result in any "damage".
lining up the door panel wire springs in the holes can be a bit challenging, lots of crawling around under the door.
Have fun