Never leave your guitar in a car!
If there is one definitive no-no for all guitarists in the history of ever, it's leaving a prized, beautiful, and beloved guitar in a car. Why? Three reasons: Humidity, temperature, and theft.
Humidity & temperature problems
As cars get hot or cold, as they fluctuate with the amount of relative humidity, they will have adverse effects on your guitar. This is like making sure that your guitar is unsatisfying to play.
Humidity can cause wood to expand or contract, and it follows that temperature can directly affect this expansion/contraction in equally disturbing ways. Imagine a freezing cold environment with a lot of humidity. Imagine that the water in the wood of your guitar freezes with all of that humidity when left in a car for too long. Pretty nasty.
Leaving a guitar in a car can really mess with the amount of contraction and expansion. If you had a guitar set up for you professionally, why mess with it's playability? Worse is when you leave it in a car and you come to find that you not only have a missing guitar but a broken window, too.
Opportunity theft problems
Many people underestimate how often guitars get stolen from cars. There are people out there who will absolutely take any easy chance to score some cash. This means that if they spot a guitar case in your car, they'll know they can sell it for a quick black-market profit.
Some people may say that they keep their guitar in their trunk, and out of sight. Out of sight, out of mind, right? I'm reminded of the thousands of bands that have been robbed of their equipment and gear in the middle of a tour, even though their gear was covered up and out of sight. It can easily happen.
You are taking a big risk by leaving your guitar in your car. Whether it is temperature, humidity, or theft, is your guitar worth it to keep safe? Is it valuable to you, unique to you? Can you afford to get a new guitar as well as getting the damage done to your vehicle worth it? Is it worth the hassle to carry your guitar somewhere safe?