Some photos of the inside of your car would help a few people suggest Ideas I should think, of the dashboard and things
I know its a pretty basic idea and you may not want to do it because it may not be that safe, depending on what you use to do it.
If you have a strong metal case for your iPAQ (the one from proporta.com is an excellent choice) you could probably attach a bit of Velcro on the back and the same for the dashboard of your car. It should be pretty strong, depending on the type you get, multiple little patches of Velcro will be the strongest way of doing it.
But, like I said, can't really suggest any more advanced, safer ways of doing it without having some idea what the inside of your car looks like
