Purchase a rear defroster repair kit from an auto parts store, thoroughly clean the rear window, apply the stencil, brush on the conductive repair compound and allow it to dry. Don't touch the uncovered glass area with any of the conductive compound during the process or it will leave fingerprints.
Locate all of the scratches or breaks in the lines of the defroster covering the window. Clean the window inside and out with a soft cloth and a cleaner that does not leave a residue. You need to be able to see clearly through the glass to accurately repair the breaks. Wipe the damaged areas with alcohol to ensure a clean work area.
Carefully apply the adhesive backed stencil over the damaged area so only the broken spot appears through the opening. Shake the conductive compound until all of the clumps of metal flakes have broken back down. Stir the bottle with a toothpick to make sure the clumps are gone.
Brush a layer of the conductive compound over the open slit. Wait for about a minute and then brush on another coat. After 30 minutes, you can remove the stencil. The repair takes about 24 to 48 hours, depending on the humidity, to completely dry.