@Weng said:
It has a magnet built into it so you can slap it up on the roof of your car. It said so on the box.
All I can say is: don't trust the box (which was cheaply made in China, just like the antenna and the glue). The magnets are to be used inside the car, preferably on some exposed metal part near one of the windows.
I worked for a while at a GPS device manufacturer and we never put the antenna outside the car, because of these kinds of problems. If the antenna doesn't fall off at speed, it will either slowly rot away in the rain and sun or the cable will break where it's passed through the door and/or window. Just slap it on the dashboard somewhere with some double-sided tape (the high quality kind) and it will never bother you again.