Common causes of rear window defroster failure include scratches across the grid lines on the rear window, broken electrical connectors on the grid and blown fuses. A scratch on one grid line can cause a partial failure, while broken connections and blown fuses can cause complete failure.
First, locate and replace the defroster fuse using the vehicle guide. If the defroster still does not function, examine each line of the rear window grid for scratches and broken connectors along the edge, or check for 12 volts of power, with the defroster on, across grid sections with a voltmeter. Auto repair stores sell kits for repairing defroster grids and the electrical connectors.