Gas door is always open, unless the central locking is engaged. You should be able to just open it. If not, it probably means the solenoid for the door lock has stuck closed. They used to be vacuum operated, but I'm not sure about the '93 models.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown