You are correct about access. There is at least one post on how to remove the sender.

The gauge is a potentiometer, and the sender supplies it with resistance to ground according to the level of its float. There is a second circuit that supplies ground at about 1/8 of a tank to turn the warning light on.

Sometimes the float gets stuck and you can free it up by soaking it in carb cleaner. Other times the whole thing goes TU.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe