To detect which line(s) is inoperative, utilize a volt meter to check for 0 V, which will be present after the open in the grid line. The check must be performed with the engine running and heated rear window switched on. Switch OFF the ignition and visually locate the damage. Proceed with checking any other lines as necessary. Once the damaged line is identified, repair utilizing the Repair kit by Permatex® ITEM 09117.