what kind of car????

many newer cars have special radiation (solar) absorbing glass (ex Mercedes R class) that even interferes with toll booth transponders.

Since the car guys assumed that all the transmitter are on the outside of the car that is where they put the antenna and now your little N900 is inside a "faraday" cage.

check your owners manual if you have a section that states you can only put toll transponders in certain spots of your front windscreen, you may have a problem