After looking at my E320 again, it looks like you can remove the cover plates without opening the sunroof.

Inside the car remove the accordian side panels. You should see the screw that holds on the cover plate on each side. Remove them and inspect the sunroof cable. If the ends are crumbled then its time for new cables.

Be carefull if opening the sunroof with the cover plates removed, they act as a cable guide.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles