To repair a back window defrost grid, use a voltimeter to determine where the grid lines have been damaged. Then, use a repair kit to reapply conductive paint to the grid lines to repair the breaks in the grid.
Apply the probes of the voltimeter to the ends of each grid line. The volitmeter should measure approximately 12 volts. Move one of the probes to the center; if the voltimeter then reads 6 volts, the grid line is functioning, but if it still reads 12 volts, it is damaged. Use a grease pencil to mark the damaged grid lines, and turn off the defroster grid and ignition switch. Some ohmmeters may be able to find the breaks in the grid without powering the grid itself up by measuring the resistance along the grid.
Clean the area of the break in the grid with lacquer thinner, mineral spirits or alcohol. Use masking tape or a peel-and-stick mask included in the repair kit to outline the repair area. Do not touch the glass directly to avoid leaving fingerprints. Shake the vial of conductive paint, and brush it over the break in the grid. Wait a minute, and add a second coat, then remove the masking after another minute.