Sounds like the retaining clips on the cover are busted or loose. I've seen a QuickTime video of the removal process on the web. It looks easy. I haven't done it myself, but I have disassembled the sunroof to replace the cables.

1. Open the sunroof about an inch or two.
2. Remove the forward trim strip. It is the black plastic piece at the front of the sunroof glass. It pops off with a wide screwdriver or putty knife. Whatever you use, only put it in about 1/8" to pop off the trim (there's a metal brace behind it you don't want to bend).
3. Open the sunroof all the way then slide the cover forward.
4. Release the retaining clips on each side of the cover and lift it out.

Let us now what you find.

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1998 E320 120K mi
1991 Miata 80K mi
2001 Ranger 109K mi