We have one car, an Accord, and our first few trips with Jackson were harrowing. I drove, Stacy sat in back with Jackson, and he stood on the backseat. That left him extremely vulnerable if we stopped short or turned hard, and made it pretty much impossible for me to see anything behind us. Stacy tried all manner of treats and cajoling, but Jackson stood tall.
We took a trip to Barker Field dog park yesterday, and we had a breakthrough. It was simple, in retrospect: Stacy sat on the middle of the rear seat instead of the end. That backed Jackson into the corner. At first, he stood anyway, like a four-legged danseur en pointe. But as soon as the car started moving, he decided that he didn't have enough room to stand, and he sat down. That got him a treat. A minute later, he lay down, head in Stacy's lap. Another treat, and a safe trip to the dog park.
On the way home, he lay down almost immediately. Woohoo!