the sunroof itself rides just below the chrome uppers of the slide. you should remove the chrome plates and clean the bottoms of them and the slide path beneath them on the outside edge.

with the chrome strips out you could also get the roof into the almost closed position and lift up on the front and remove the front headliner clips and slide it out over the roof. this gives you access to the clip that attaches the sunroof to the sunroof cable.

once the clip is removed you should be able to carefully slide your sunroof up and out of the opening in your roof. give the plastic slider pads a good cleaning and lubing (or replace like my case). and also clean and lube the slide rails as far back as you can manage and lube them as well.

with the sunroof out of the way you can also check the cable for smooth movement.

good luck

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.