I just successfully fixed the problem on my 88 TE, using contact cement. The trick is to throughly clean the surfaces with acetone and Q-tips, clean away all foam as you need vinyl/leather contact to a hard surface, without any mush in-between. Buy fresh, solvent based contact cement - tubes are available. Spread thin coatings over both surfaces, let dry, then do it again to be sure you have covered all voids. Let dry until paper won't stick to a glued surface, then squeeze the surfaces together and hold them with many clothespins - one per inch, for a couple of hours. Don't overly stretch the material when squeezing them together as that will add additional stress on the glue joints.
If they simply will not stick, you could slit appropriately sized plastic tubing and slip it over the edge to physically hold the material. You might find it in matching colors to your upholstery.
Good luck!

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'76 240D-Sold
'78 240D-Sold
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'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
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