Had the same problem on my 85, might be the same thing.  There is a latch
with a spring on it on the back of the body that snaps into the rectangular
plastic mound on the door as it opens.  It has a spring on it, with a
compression(?) washer holding it in place.  Somehow, my washer fell off, the
spring flipped up, and the latch wouldn't catch.  A new washer held the
spring on, and I drilled a tiny hole in the post and tapped a pin in to hold
this washer on permanently.  It's easier to access if you take off the narrow
body panel above the sliding track (a couple screws).

Bill Marshall
85 GL Tiico
PHX AZ