I'd say to leave them in the carrier and have it up front with you if the front passenger seat can accomodate it.
They'll feel safer seeing you there and you can talk to them and have a feel for if they ~really~ need out or are just being bratty. :-P
Ya know...that thought hadn't occurred to me. Duh. I could probably even put the litter box on the floor there. Then the back could really be packed.