When I toilet trained my first son, I took a week off of work in order to potty train him. I switched him into underwear and swore never to put him back into diapers. I also did not believe in training pants because they are no different than diapers anyway, and I didn't want to confuse him. But what about the car seat? Not wanting to risk having to clean the seat from accidents, I put a small change mat on the seat with a prefold on top. My two piece system was quite annoying because the mat and the diaper would not stay together; one was always shifting out of place.
When it came time for the car seat to be protected from my second son, I created the Child Seat Protector. It is like a pocket diaper for your car seat that can be stuffed with a prefold! We have used it for over a year in both our cars, and it works. You can get an insert for it too that snaps into place. This is great if you didn't use prefolds, or simply want an insert that fits well and will stay in place.