This is what my husband and I got for Butters.
http://www.amazon.com/Solvit-Tagalon...+seat+tagalong
He loves it! This got us through a 24 hr drive from NY to Texas. He refused to sit there with just the harness secured to the seatbelt. Somehow, this made him feel more comfortable and less restricted. He enjoyed looking out the window and being a bit elevated off of the seat. There is an attachment at the bottom of the carrier that latches onto his harness to keep him secured. We assemble it in the back seat of our little sedan (you do, however, need back seats with a head rests and fold-down/single standing capability to secure the booster properly). There are some sample photos on the Amazon link, to include assembly in back seats