On my Z3, the plastic rear window is sewn onto the convertable fabric. The cosmetic/weatherproofing vinyl ring around this seam is popping off. What’s the best adhesive to use, keeping in mind a ragtop repair place may eventually replace the aging window?
I’d try silicone adhesive, because it’s super-flexible.
Both surfaces have to be completely clean or the bond will come loose. If you can’t clean old adhesive and dirt off, get it done professionally now.
Can you post a photo of the damage?
Might help.
I’d have the shop fix it right
Looking into this a bit further, it’s just a cosmetic trim. Here’s a picture. 
I don’t think an adhesive is going to work in this application. The easiest fix is to cut it off, but you might not like the way it looks. Another temporary fix is to tape it on. Maybe some brown take that matches on the inside of the flap and just covers the bottom of the detached trim. Another possibility is brown or clear thread sewn in a spiral over the trim and sewn through the flap. The thread must be waterproof, of course. I’m brainstorming and hope that these ideas might help you find something short a a professional repair.
There’s a gadget called a plastic welder that might work.
JB Weld makes an adhesive for plastic too.
I like the welder idea, but not the adhesive. Adhesives don’t hold thin sections together well and the length of the detached area makes it even worse.
The plastic welding idea could be tested with a soldering iron. If you have one, try welding a spot close to the ends where the trim is still attached. Be careful. It is very easy to melt too much of the trim and hurt the looks.
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Thanks for the photo. That does help.
Question: have you asked a shop that does convertible roofs what it’d cost to fix it right? It might not be as expensive as you fear. They may know something the average schmuck like me doesn’t. Hey, asking is free!
Don’t mess around. Just have it done right by a convertible top shop
Another thought…has that strip shrunk in length, i.e., if you push it back into place will it easily stay there? If it’s shrunk, it’ll have to be replaced. For a stop-gap fix, you’ll have to cut it at, say, just above the lower left corner for any repair to have a chance of holding.
I guess I was looking for an answer like: use 3M Black weatherstrip adhesive after cleaning the surfaces with the proper solvent. https://tinyurl.com/yaqgwn9j
Replace it.
http://www.roadstersolutions.com/page6.html
Also, the only time a convertible top should be down is when the vehicle is driven.
Otherwise, you get this type of damage while the top is folded.
Tester
For some reason, I’ve had better luck over the years with the yellow weatherstrip adhesive.
Shrunken old plastic needs to be replaced, not repaired.
I’m a big fan of Haartz Corp. They make most of the convertible tops sold in the U.S. and Europe. Their website says “We want to hear from you. Would you like more information about our company, products, services or vintage car restoration?” You may want to send them the question here. I would do it for you, but you would get the answer faster this way. Let us know if they respond.
I had the same problem with my Z3. After some research, I got some Barge All Purpose Cement. However, before I had a chance to try it, enough of the molding came loose that I just pulled it all off. That turned out to be the better solution. It appears to be mostly cosmetic. Removing it looks better than a sloppy repair job would have. By the way, the window can be replaced without replacing the entire top. If you look at the window from the inside, you will see that it is held in with a zipper. I have read that it is not a DIY project, but if you have time and patience, it might be worth giving it a try.