My roadster's time finally came. I had it parked outside in the sun while doing some repairs in the garage, and later that night I discovered the rear window had separated at the top. I live in Michigan and the car has been garaged its whole life (and usually covered as well in the garage), although it spends the summer with the top almost permanently down and the 4-5 months of winter in an unheated storage unit with the top up.

Visited a local upholstery shop who had repaired my motorcycle seat and confirmed it was not repairable (reason below). They ordered a new top from somewhere in CA in the original German black fabric with a glass rear window with defroster. Should be here in ~3 weeks. Top is about $2100 and labor $770. This is not a DIY deal for me - I want if professionally repaired and good for another 20 years.

The rest of the story (reason): First, he told me a convertible top life is 12-16 years so I did pretty good at 20. Secondly, he told me the reason the glass fell out was not that the adhesive failed but that over time the top shrinks and as the fabric becomes "smaller" the failure point is the fabric pulling away from the rear window!! At this point I recalled that putting the top up a couple days ago was more difficult than usual (the car was parked outside during the garage repairs). He also shared that my habit of leaving the top down all summer probably contributed to the shrinkage. He recommended putting the top up periodically so that it is stretched out gently by the mechanisms rather than shrinking for months and then being subject to closing. So the message I took away is do not keep the top down for extended periods. My plan is to put the (new) top up at least every other week in the summer (it already spends the winter with the top up).