I mean it all depends on the length of the car journey! If it’s a journey where you can stop easily and get out in side of road for a wee I’d do it that way if it’s a long trip on a motorway and not easy to stop then puppy training pads or the bed pads in the car seat would be a good way to protects it! With maxi any clothing one the bottom is an issue I dunno how we’re going to conquer leaving the house
On shorter trips in the car I just made him go to the toilet before we left and then relatively soon after whenever we got to where we were going. On longer trips at first (1hr+ to my parents for example) I put a pull up on him in the very early days, but it was never needed so I stopped and then just did the same as the short trips, and made sure I had the travel potty! I had one close call in a drive thru after nursery one day, where he told me he did a wee before we left and then announced he needed it mid queue. Longest <10mins of my life 😅 But that was fine and he held it until I could get him to the toilet- though that was about 2-3 weeks into training.
If you are able to plan the car journey that will help, avoids fluids for say half an hour before or straight out after a potty trip. I honestly did it without thinking the first couple of times and then just winged it thereafter, we’ve had an accident in the car seat once, didn’t take long for the covers to dry once I washed it and was really easy taking it on and off. Good luck! X