I recently found that the tailgate would not lock in my 1989 240. Not by key or central locking. I removed the carpeted panel on the tailgate and inspected the mechanism while turning the key back and forth. There is a thick pin which when locked is supposed to move and clear the mechanism which allows the tailgate to be opened. It was basically not moving over far enough so I removed a metal rod which looks like a clotheshanger and is bent in two spots and bent it at both spots just a little bit more (essentially shortening the rod). I replaced and tested it and now the pin clears the opening mechanism. Very easy fix. Hope this helps someone.

Test your tailgate now and then to make sure it is locked because I bet mine was broken for a while. I always take for granted that the central locking locks all the doors. It is easy to see the doors lock but I never really check to see if the tailgate is locked as well.