Strip the ends of the cable and use a multimeter to check that these leads are no longer shorted

If your continuity tester still shows a short, the cable may be damaged elsewhere, check again for any signs of damage - maybe you just need to shorten it a few more inches

If you can't find the source of the short outside the adapter, you're going to have to open the adapter itself and continue there (see other guides for details on how to accomplish this)