Now, don't take this as truth, but I believe that you are able to leave the iPass basically anywhere in the car and it should be picked up by the tolling system. That's why when they send them to you in the mail, they wrap them in tin foil to block out the radio signals from activating the units in the back of the mail truck.
There is a tiny battery inside the unit which allows it to activates, receive the rfid request, respond with it's ID and then shut down all within milliseconds as you're driving down the highway. The radio signal is at such a bandwidth that the it shouldn't really have a problem transmitting through anything in your car. The reason that they want it out is that depending on the makeup of the car, the metal structure could cause interference. There are also some cars that have a coating on the windshield that also can interfere with the signal, and that's why they have the license plate option.
Yeah, I'm a nerdy engineer. So be it...