Rynnakat has a good idea w

Rynnakat has a good idea with the bike rack thing. If you are going to use duct tape as in her latter suggestion I would suggest putting a small bean bag under the camera. For a more pro approachtake a look at solutions like the one in the link below. Also FYI I would remove the rear view mirror from the inside of the car. usually just requires an allen wrench:

http://www.markertek.com/SearchProduct.asp?item=DELV%2DSUC&off=6&sort=prod&skuonly=0&search=suction&pagesize=20