I had the same problem on our '79 300D, right front door.I'm assuming the design is similar, where the map pocket had a molded-in hook that does thru the door panel and is secured by a steel clip form the other side. The fix goes as follows, assuming you still have the broken end rattling around inside your door. 1) fire up the hot melt glue gun. 2) glue the broken end back on the map pocket, using just enough to hold in place. 3) pass the map pocket thru the door trim panel, holding the map pocket tightly against the panel. 4) flood the entire backside of the hook with hot glue until it is fully covered. 5) continue applying pressure against the map pocket until the glue cools and sets. We have had no trouble all summer, (fix was done in May). Bear in mind that the map pocket should not be loaded with phone books, etc. after this procedure, but so far it's still working.
'79 300D, 206K