I didn't know 83's had a lock. Open the trunk, and look behind the coverings toward the back of the fuel door. You can unlock it from there. My 84 300sd didn't have this feature, but both of my 89's do.
you have to remove window crank and door handel first
The switches are on the door, by the handle.
My 84 D turbo has the lock vacuum pump in with the spare tire .
there is no release you just push on the door near the hinge and it opens
Foot in the Door - 1983 Pilot 1-1 was released on: USA: 28 March 1983
Foot in the Door - 1983 Jonah Moves Out 1-2 was released on: USA: 4 April 1983
Foot in the Door - 1983 The Big Breakthrough 1-4 was released on: USA: 18 April 1983
Foot in the Door - 1983 Jonah the Babysitter 1-5 was released on: USA: 25 April 1983
The 1985 Mercedes 380 SE has a gas door release latch located on the driver's side floor board near the hood release lever. If the gas door release latch is no longer operable, the cable to the door might be broken.
http://www.peachparts.com/shopforum/diesel-discussion/215532-vacuum-door-locks-not-working-1983-300d.html
Well on my 1991 Mercedes Benz 190e the fuel release door is automatic. All you have to do is make sure all the doors are unlocked with the master key and then, go to the fuel door and lightly press in on the left side of the fuel door and pull from the right and it should pop open.
Foot in the Door - 1983 Jonah Takes a Job 1-6 was released on: USA: 2 May 1983