In the 126 there is a vacuum actuator that locks the gas cap door when the doors lock. Assuming the other doors lock/unlock, you might have a bad actuator or actuactor linkage at that "door" only. There is a manual release (cable loop pull) on the inside of the trunk...filler neck side. The 201 may be similar.
Bruce (I could be wrong) E.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)