I own the same model of vehicle as Waymo is using (2017 Pacifica PHEV). It's cool to see how little they've had to visibly modify the car (apart from the sensors)—the interior looks 100% stock, other than the row of buttons on the ceiling. Given the level-1 autonomous features in the vehicle (self-parking, etc.), all of the integrations are probably done over CAN-bus, with no additional hardware required (again, apart from the sensors and the computing power).