I had a similar problem with my 79 300d. The p/o had gooped silicone all over the outside to seal it (indicating a new seal is in order). When I finally removed the mess, the rear portion of the sunroof dropped in and I couldn't get it close again. So I took it all apart. It was easy. I found that many of the small metal parts that keep the roof on track were bent up, and the little plastic feet/bushing things were pretty well beat. I straightened up the bent parts and put it all back together with a little grease on the tracks, manipulated the salvaged feet/bushing things back in their place, and now it closes and opens fine.

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79 300D 238K mi