The rear-window defroster on your back glass is only painted on. Sometimes it can get damaged or even get a disruption in the electrical flow. Patches of frost on your back glass would be an indicator of a problem.
(1) Check your fuse, which located in the fuse box going to the rear-window defroster unit. Use a voltmeter to see if both sides of the fuse get a sufficient reading. If you can’t get a reading, replacement of the fuse will be necessary.
(2) Check the terminals on the defroster grid, which are usually located at the top or sometimes the bottom of the grid. Sometimes the terminals get broken. Check both terminals and if they are broken, they can be put back together with glue. Be sure that the broken ends are placed together firmly so as to receive continuity. (Look at your wiring diagram located in your automotive manual.)
(3) Locate the wires plugged into the back of the defroster switch for the rear window. Be sure the fuse is plugged in. then, disconnect both wires leading to the switch and touch both ends of the wires. Hold them together for a few seconds and the defroster should not come on while the wires are held together. (This will mean that you will need a new defroster switch.)
(4) Inspect the wires under the dash leading away from the defroster switch and if they are intact, check the defroster grid on the rear window for any breaks. If any breaks are found but the ends are intact, you can apply some glue on both ends of the broken pieces of the grid and stick them back together.
(5) If you find a broken piece of the grid that is missing a piece then, you will need to buy a defroster repair kit (available at an auto parts retailer) You will find conductive paint included within the kit.
(6)Clean off the area of the break with alcohol and apply the paint to the area that is broken.
(7)Let the area dry for 12 to 24 hours, then your defroster will be ready to use.
By following these steps outlined above, replacement of the entire back glass is not entirely necessary when the rear-window defroster on the car’s back glass gets damaged.