I mentioned a while back, a contraption I made to go in the car door. It was a little difficult to explain so these pics may help. It just wedges down behind the door handle and the "give" in the door trim below the window holds it really tight. I must add that I normally use a large Benbow ball and socket head. I often drive along slowly with the camera attatched.
If you like it, you will have to go out and buy a Citoen Xsara !!