As long as you dry him after the soak it won't make him any colder. I would keep the car warmer if you can, though you don't want him to overheat, but depending on where you live I wouldn't think that easy to do in the middle of winter haha. As long as he stays above 75f there shouldn't be any concern. Definitely bring a thermometer or, preferably a temp gun so you can check temps on the fly. 7 hours isn't too much either, there's people on here who have had power outages for a lot longer than that and managed to keep their snakes warm enough. I would focus on minimizing impact from the bumpy road, and maintaining temps, and it should be fine. Make sure the pillow case and cooler are very secure as well! They are great at escaping when you don't think it's possible.