From inside the trunk, remove the liner on the side of the fuel door. Now look up along the wall in the area where the fuel pipe enters the car.
The device you seek will have a vacuum line attached to it and with a little effort you may be able to pull back the spring plunger thingy while someone on the outside opens the door.

Your problem could be as simple as a bad actator.(This is the vacuum thingy that moves the little shaft into the locking or unlocking position on your fuel door). Or as difficult as locating the break in the vacuum line that supplies vacuum to the actator. Usually if this is the case, you will have an issue with your central vacuum locking system too.

If your car is a w126, the vacuum pump for this system is in the spare tire well on the lower right side. Not sure of other models though.

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