Yes, I have done some of this (shooting thorugh auto windshield). One thing I can ensure you, you will need a polarizing filter. That way you can eliminate the glare caused by the glass.
I have had pretty good luck by using a lightweight tripod and keep the legs closed up and placing the tripod on the floor and running it up between my knees. Now that assumes you have a driver for the vehicle which in my opinion is the only safe way anyhow; as someone needs to pay attention to the driving, while someone else is paying attention to the camera and the scene being shot.
I have also used a sandbag and placed it on the dash and then lightly help the camera on top of the sandbag. Yo can buy them, but you can also make your own pretty easily.
I agree with the other posts in that I would not trust a suction cup only type mount outside the vehicle. The Gecko mount would probably do OK for a very lightweight camera inside the car. Be sure the glass is clean and moisten the suction cup before placement.
For outside the vehicle I have seen mounts that use a clip that fits under the hood and 2 suction cups. That would be trustworthy outside, as the strap with clip under the hood will keep the camera from falling should the suction cups loose their grip.
Artcile on the basics of auto mounts here: http://www.videomaker.com/article/13050/