There are plenty of solutions for traveling shots and filming into cars from the drivers and passengers side but one to keep in mind for cameras of all sizes and weights is the Modern Deluxe Side Mount/Hostess Tray.
Modern Studio Equipment makes a Hostess Tray / Side Tray which allows attaching a shooting platform and filming into cars a breeze. With many points of adjustment it makes it very easy to mount the Modern Studio Equipment MODERN DELUXE SIDE MOUNT/HOSTESS TRAY to a variety of car doors. Expect to pay between $740 and $900 for the setup.
For more see Modern Studio Equipment for other grip and rigging equipment.