General mountain dulcimer or music discussions
Kandee, changes in humidity rather than temperature are the challenge. Wood takes on and gives off moisture continuously and will naturally mimic the ambient air conditions. The wood will give off and take on moisture until it is in balance with the moisture in the air. As wood takes on water it expands and as gives off water it contracts. Dry enough conditions, for a long enough period of time, can eventually cause cracking. The more stable you can keep the humidity in the instrument, the better.
Wood doesn't expand and contract much due to air temperature.
The good news for you is that the process is usually slow, so you likely won't hurt your dulcimer by leaving it in your trunk for a day. I wouldn't leave in the cabin of the car where temps can get really high (well above 120). That could cause issues with the glue joints if it was made with a traditional hide glue or something like that. If it was made with a contemporary wood glue (which it most likely was) you probably won't have an issue.