One of the automotive conveniences we've come to rely on is the rear window defroster, or defogger. But when the defroster stops working, it's not just a nuisance, it's a safety issue. Without rear window visibility, driving, and especially backing up, becomes a major hazard. Fortunately for drivers, repairing a rear window defogger is a process most anyone can accomplish in about half an hour with little expense. And with lives potentially at stake, it's a repair well worth making.
Visually inspect the heating grid attached to the inside of the rear window for breaks in the horizontal lines. Any break in the filaments in the grid will cause a partial failure of the defroster, leaving a portion of the window fogged or obscured.
Use a test light or voltmeter to detect breaks in the filaments that aren't visible to the naked eye. Touch one lead of the light or voltmeter to a grounded metal part of the car and the other lead to a spot on the heating grid. If the light doesn't light or the voltmeter doesn't register a reading, that indicates a break in the filament.
Place a piece of tape on each side of the break. Align one piece just above the filament, the other piece just below it, leaving a thin space through which the filament passes.
Apply the filament repair liquid with its applicator onto each break you've identified. Be sure to thoroughly cover the broken spot. Allow the repair to fully cure per the instructions on the bottle, then carefully remove the pieces of tape so they don't rip the filament repair.
Inspect the vertical lines on the heating grid if you've discovered that the entire heating grid fails to work. Any break in the vertical lines will cause complete grid failure. These can be can be repaired in the same way as the horizontal lines. Check to ensure that the metal power tabs on each side of the grid are securely fastened to the vertical lines, as a break here will also prevent the entire grid from heating.
Reattach the metal power tabs to the heating grid as needed by removing the tab from its connector and repairing it using the epoxy found in the heating tab repair kit. Be sure to allow the full curing time indicated on the kit before reinserting the tab into the connector to ensure that the circuit isn't broken.
Check the power switch and wiring to the rear window defogger if there are no breaks in the heating grid and both of the metal power tabs are securely connected to the grid. Replace the switch and/or wiring as necessary.