I'll start out by saying that I'm way too paranoid to leave my stuff in the car on hot/cold days.
But ..... about a decade ago I worked my day job with a guy that was a heavily gigging bassist on nights and weekends. His primary instrument was an early 60's Fender Jazz with a home-grown oiled natural refin. He left that bass in the trunk of his car, in a generic plywood case, every day of the year. He swore up and down that he very rarely had to do truss rod tweaks. I didn't believe him, so we walked out to his car on our lunch break - on a stanky humid North Carolina summer day, 90+ degrees in the middle of an asphalt parking lot. He pulled it out of the trunk and handed it to me. Played like butter!
Moral of the story: I dunno.
But ..... about a decade ago I worked my day job with a guy that was a heavily gigging bassist on nights and weekends. His primary instrument was an early 60's Fender Jazz with a home-grown oiled natural refin. He left that bass in the trunk of his car, in a generic plywood case, every day of the year. He swore up and down that he very rarely had to do truss rod tweaks. I didn't believe him, so we walked out to his car on our lunch break - on a stanky humid North Carolina summer day, 90+ degrees in the middle of an asphalt parking lot. He pulled it out of the trunk and handed it to me. Played like butter!
Moral of the story: I dunno.