It sounds very much like you are suffering with a form of sleepwalking, which has the medical term of somnambulism. This condition is quite common and affects people in very different ways, such as from a simple shift of position while asleep to carrying out very hazardous activities such as driving a car.
It normally affects children and younger people and may subside altogether in adulthood, although this does not apply to everyone. Many experts agree that sleepwalking in adults can arise as a result of stress and anxiety, so it is important to identify if these issues may be to blame and find ways of resolving the problems before you are harmed whilst sleepwalking.
Prevent Access During Sleeping Time
With regard to the immediate problem, you must find some way of preventing yourself from getting into a car when asleep. Is it possible for someone to hide your car keys until the next morning or keep the front door locked while you are asleep? Obviously your safety and the safety of members of the public are paramount, so you must not be able to have access to a car when you are asleep until your sleepwalking has been resolved.
Try to Find the Cause of the Sleepwalking
Finding out the cause of the sleepwalking may be the first step to finding a cure. It is possible that genetics play a role so if you have a relation who has also suffered with sleepwalking in the past, this may explain why you have started to do it. Medical issues may also be contributing to the problem.
Please go and see your GP and explain your concerns and find out if there is an underlying medical condition that may be causing you to sleepwalk. You may be able to seek a referral to a sleep specialist who will have expertise in this area and be able to guide you through the problem offering cures or ways of preventing you coming to harm.
Some sleepwalkers use alarms that are activated if they leave their bedroom during the night; these can be useful for some while others do not actually hear the sounds when they are asleep.
If you do go and see your GP or a specialist try to take some video evidence with you; can someone video these episodes (and stop you leaving the house) or can you set up a camera for one night to try and catch the movements?