That means you can pull LT (360) or press L2 (PS3) to aim, and you reload with X or Square. Now, as the passenger of the car, the controls are identical to walking, not driving.
He'll always prefer to enter the same vehicle, and if you switch, it might take him a bit to figure out what you're doing, so you don't have the time to be choosy. You can always boost another car as long as your ally is alive, and he'll automatically get in behind the wheel. Unlike previous Saints Row games, if the car is destroyed, you don't necessarily lose. Once your ally stops, you can either hop out the car as well, or stay in to use the car as armor. Rather than destroy their vehicles-which will just cause more to spawn right away-aim for the drivers or the tires: take them out, and it stops the pursuit, though of course more will spawn in eventually.
The cops and/or rival gangs are instantly on you here. Your job is to cover his back and shoot anyone who tries to stop you. In Trafficking, you'll ride as the passenger as your ally drives around and does some business.