Fuel filler flap lock is easy to get at, it's in the storage compartment on the right side at the rear on the wagon. Just unlatch it, the lock is right behind the power antena motor. Most likely you have a broken plastic line, but its possible the diapham is gone. Easy to check once you're in there.
There is a red lock button for manually unlocking the filler door, and check to see if the locking tab is bent over, trapping the lock actuator. I found this to be a problem on both the TE and the 300D.
One the sedan, it is necessary to pull the cover on the right front side of the trunk to get to the filler door lock.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!