The windshield placement is necessary so that the overhead readers can scan the transponder. There are two approved transponder locations that maximize device readability. The first is at least two inches but not more than three inches from the top center of the windshield. The second is two to three inches from the lower part of the windshield, especially on trucks with visors and/or overhangs. If you have a right fender spot mirror, mount the device to the left of the center of the windshield to eliminate any mirror blind spots.