Years ago, when the National was first held in Alice Springs, lots of people from the temperate climes of southern Australia ventured into the great outback with their instruments, with no understanding of what heat could do to them. Almost every accordion ended up with a few reeds clattering around inside, some wooden flutes developed cracks and the occasional concertina reed frame slid out of its slot.

Most of the concertinas were easily fixed as not many of the frames could fall away from the pan. Accordions provided more of a challenge, as the reed's pitch away from the block can differ from its pitch when in position. Thankfully most are marked. Providing you could tell which reed was which when you found it among the bellows folds in the accordion, all you had to do was ensure no loose wax was able to fly around, put the frame in place on its block and apply a warmed table-knife blade to remelt the wax. None of them got hot enough to melt the animal glues/resorcinol/etc that attached wooden parts or bellows parts to each other.

Another folkie visiting the Longford festival in Tassie was not so lucky. He left his fiddle (in its nice black case) on the parcel shelf under the rear window of his car for a couple of hours. It had been in tune when he left it in the car in the sun but, when he next opened the case, he found he had acquired a complete fiddle kit.

I used to be a photographer and learned never to leave a camera (lots of black) or a camera case (usually black) in the sun, not even in the Antarctic. When I got a robust stage case made for my concer I made sure it was rather reflective. But I still don't let the heat get to it for just the reason you mention.
Cheers, Rowan