Storing a firearm in a vehicle is convenient, but in many case not advisable. Understand that you can be held at least partially culpable for the actions of a criminal if a firearm is stolen from your vehicle. If you do decide to carry a firearm in your vehicle it is a good idea to add another layer of security.
Another issue with keeping a firearm in your vehicle is the risk of corrosion. Moisture in the winter, as well as temperature swings can leave a layer of water on the firearm. This can be very hard on a firearm, especially if it is not regularly inspected.
Always be safe and remember the 4 gun safety rules!