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I am afraid you have to purchase the whole Nav gadget, the drive as well as the DVD itself. I'd imagine the drive would not read English. The only people that may give you a real no-fee answer is the Japanese dealer you got your car from or may be Lexus Japan. Good luck.
The difficulty is the buttons on the left and right of the screen are in Japanese as is the handbook. Can you assist with the switch and the steps from there