It’s the latest in high-tech advertising. Little boxes are being installed along roadsides that detect which radio stations drivers are listening to. “The monitoring aims to help retailers choose where to advertise by giving them a snapshot of which stations consumers tune into as they drive by their businesses. The most enthusiastic MobilTrak adopters: auto dealers, who generally believe that 80 percent of their business is with people who live or work within 10 miles of a given dealership.”