Guess what? Just like kids and animals should not be left in the vehicle for extended periods of time. It doesn’t matter if it’s hot outside or cold. Either weather is bad for your instrument! Even while left in a hard shell case, guitars have been known to warp (even to the slightest) when left in a hot vehicle. They can crack in cold temps. Think of your instrument as you do your child or pet; take them indoors with you. They are valuable, afterall.