Roxie pants in the car too. She always wants to sit on the driver's lap.
I purchased this a few weeks ago:
Pet Lookout Car Booster Seat - Automobile Travel Gear - Pet Travel Gear
However, despite following what little directions it came with, I had a bad experience with it. Roxie tried to get out of it while I was driving and wound up hanging out the side by her neck! I had to pull over quickly in order to fix the situation. Needless to say, I returned it.
I use this and it works just fine. I can also use it for walks.
Amazon.com: Easy Rider Car Extra Small Harness...