The good thing about 35+ yr old vehicles - simplicity. Take off the inner panel and check it out. Yes could easily be the latch assembly or rod connection problems. These things are often a combination of bent/weak rod and hard-to-move parts, etc. The panel is easy to get off after the armrest, window crank, etc. Fiddle around and you will see how it all works.
Removal of the latch assembly is pretty easy also.