Don't leave your dog free in the car, road traffic rules forbid it: they could bother or distract the driver and, in the event of an accident, they wouldn't be protected.
Kennels are the best and most common way to travel by car with dogs. Despite what you may think, dogs like them because they remind them of a haunt. Additionally, in the event of an accident, the kennel protects your dog.
First steps: put the kennel in the car without starting the engine. Your dog will start getting used to it. Then, once they are settled, you can go to a place they like, for example, a park near home. It will be easier to get them in the car if they associate the kennel with a reward, such as going to play in the park.
If the kennel turns out to be a no-no, you have two alternatives permitted by the law: the first is to put them on the back seat with a special safety belt(sold in pet shops); the second is the boot with a separation net. In both cases you can start slowly to get them use to it, as suggested for the kennel.
Keep the temperature inside the car cool, don't turn on the air conditioning too high. Avoid keeping your car windows completely open, your dog may try to jump out or get a bad ear infection.
Let your dog out of the car every time you stop: take them for a short walk and then give them water. Don't feed them, unless absolutely necessary (e.g. if the journey is very long). If you do feed them, give them dry food. Don't leave your dog in the car in the sun.