People use a variety of devices to accomplish this from different parts of the vehicle. Woods brand suction mounts, Cardellini clamps, baby spuds, grip arms, ratchet straps, cheese plates, trays and other mounting surfaces are basic components for mounting heavier cameras.
I tend to over-engineer my car mounts. Using three or more points spreads the load widely across the vehicle's surfaces, thereby reducing vibrations. Take additional purchase points on the camera's top handles or to baby spuds threaded into the cameras 1/4 - 20 taps.
This site has all the major implements you can imagine for car mounts and other process work.