The unit should also come with an adhesive disk that fastens to the suction cup then double face tapes to your dash.
You could also buy the weighted mount like this.
Keep in mind that all GPS is "line of sight". In other words it has to be able to see the sky and must be seen by a minimum of three Satalites in order to work.
The signal cannot be recieved through solid objects such as the roof etc.
If your are going to mount it anywere else, Other than the windshield area you have to buy an external antenna like this.
Have you thought about buying one of the Nuvi ' * ?
They are less bulky then the Street Pilots.
Hope this helps,