TOKYO—When selling car navigation systems, Japanese retailer Autobacs touts a special add-on: kits that convert the mapping gadgets into televisions, so drivers can watch their favorite shows.

"Even While Driving—Enables Digital Viewing and Navigation Operation," boasts the package of one kit resting atop new navigation systems in a Tokyo store. Boxes of other converters show "before" and "after" images of a navigation screen transformed into a TV depicting a soccer game broadcast.

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