Yes, it is possible to grease the sunroof cable (there's only one) without completely removing the sunroof. You do have to remove a bunch of stuff though, to get the cable out. If your sunroof is not moving smoothly, then you probably have sticky tracks. Never hurts to grease the cable, but the tracks need cleaning and lubrication more frequently than the cable does. If the sunroof moves slowly, but smoothly, then you may just need to adjust the drive clutch.

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'79 240D