Start small
Comfortable car travel means secure restraint, fresh air, and breaks that suit your dog.
There is no need to rush. A predictable, manageable step is usually more useful than trying to solve the whole situation in one day.
Try this next
Use an appropriate travel setup, plan regular stops, and never leave a dog alone in a hot car.
Notice what helps your dog stay comfortable and engaged, then repeat that version before making the situation harder.
Keep it kind and practical
Keep the next step easy enough that your dog can succeed. If you are worried about safety or feel stuck, pause and ask a qualified reward-based professional for help.