Of the two cameras you mentioned, I'd use the PD100 for this shot.
As I said, relaxation is the key. Breath slowly and hold the camera firmly but loosly. To get the idea, try holding your hands (without the camera) in the position I indicated and move them slowly up and down (as though they were attached to a big spring). You'll notice that you can move them smoothly and without shaking.
Do the same thing when you're holding the camera and you'll be able to absorb most bumps.
To check your shot, open the side viewfinder and tilt it upwards so you can control your shot just by glancing downwards, every now and then.
Once you've found the appropriate height (one that allows you to see the road without framing the dashboard, position your hands close to your chest (you don't want the muscles in your arms to tense up) , hold the camera firmly while keeping your hands loose so they will cushion the eventual bump and off you go!
Remember to stay relaxed because if you tense up you won't be able to cushion the camera and compensate for the bumps!