If its like a W126, there should be some little plastic clips on the liner that clip into small plastic holes in the sunroof itself. I have mine pulled out, and I have not fixed the roof yet, so I have not yet learned to put one in. I hope this helps.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual