Reading your post again it sounds like your cable is too short. When the roof is all the way closed the cable should still be sticking out of the back of the gearbox a bit, and you should not be able to remove the tube at the rear of the gear box due to the cable extending into it slightly.

This lock mechanism is simply the cross bar that the cable attaches to as seem in pictures 3,4,10 and 11. The cable pushes the roof closed, then continues pushing a bit to raise the back of the panel.
If you can get your roof open a bit then pull the headliner off and take a look; if you can’t try popping the clips at the front off and gently pull down in the center of the headliner at the front and rear to free the sides (into a taco shape). To remove the headliner with the roof closed you may have to sacrifice the headliner...

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine