Be careful of whats being shown through the windows. Careful of billboards and advertisements, people, and random stuff you dont want in the background.
One time in a feature we used a Hummer H2 with the DP sitting shotgun, the sound engineer, director, and grip holding a reflector all in the back where we had removed the back seat.
IF at all possible use an external monitor either wireless and close by, or in the car so your director can make sure your shots dont include anything you dont want.
Another thing to look out for is justaposition, making sure there arent poles or wires going through your talents heads in the background.