How do you open the door to the fuel tank on a 1980 Mercedes 300?

The fuel door lock is controlled by a vacuum valve (as are the doors locks and many other components on these models). If the car is unlocked, you should be able to open the door easily with you fingers. If the car is unlocked and the door will not open, the valve has failed and needs to be replaced. If you open the trunk of the car and look on the right hand wall of the trunk, you will see a little "door" right behind where the fuel door is. Remove this door and there is a manual release for the door. Hope this helps!