TOKYO — In the year since the Japanese authorities arrested Carlos Ghosn, the fallen Nissan boss, prosecutors have seized laptops, hard drives and reams of files from the automaker and its executives as they build their case against him.

Now, Mr. Ghosn’s defense attorneys want access to that information as they look for evidence that could clear their client. But prosecutors have declared thousands of the files off limits. Nissan, they say, considers the information too sensitive, and prosecutors agree, refusing to provide any details of what they plan to withhold.

So far, judges have sided with the prosecutors. The legal clash is now headed to Japan’s top court, as Mr. Ghosn’s lawyers make a last-ditch attempt to get access to the files, or at least to find out what they are being barred from seeing.

Their appeal, filed on Oct. 17, is the latest twist in a case that has riveted the global auto industry and raised fundamental questions about the fairness of Japan’s justice system. The decision not to disclose the files, Mr. Ghosn’s attorneys argue, highlights the cozy relationship between prosecutors and Nissan, as well as a Japanese legal system that too often favors prosecutors.