We used the crib pads (things that go under the sheet) folded up in their car seats after we had a few accidents in the car. Like someone else mentioned, when they are first learning, if they say they have to go, they have to go NOW so accidents in the car will happen. It is a total pain to take apart and wash the car seat, so using the pad works wonders!
And don't forget to ask when you are out and about if they have to go to the bathroom BEFORE everyone gets in the car. We would be leaving a store and it would always seem like one of the boys would have to go after I got everyone strapped in their seats, then I would have to take everyone out again, go back in the store, etc. It is a learning process for us to remember to ask them all the time too - you forget they need to go sometimes!