Q: I just noticed my rear window defroster has a big gap in the middle that doesn’t defrost. It’s about 2 inches tall. I was told the window needs to be replaced to fix this, and it’s really expensive. Is there a way to fix it?

A: Yes, you should be able to fix the defroster, and at low cost. The inside surface of your rear window contains a delicate network of conductive stripes that have been silk-screened then baked onto the glass. A conductor on one side of the window receives voltage, and another on the other side is connected to the vehicle body. Each stripe bridges between the two conductors and acts as an electrical resistor, giving off heat as electric current passes through it.

Rear window defroster stripes are easily damaged by people, pets or cargo. Sometimes the entire window can fail to be heated if the fragile wire connections break away from the window conductors. That is more difficult to fix.

The first step in repairing bad stripes is to identify specifically which ones are faulty. During a frosty morning, look closely and note which stripes are inoperative. If you can’t tell, an inexpensive automotive test light will be needed to find the exact spot to repair.

Next, pick up a defroster repair kit from an auto parts store, along with a test light, if needed.

Clip the test light’s alligator clip to a bare metal vehicle part, such as a door or hatch latch striker. Now, with the ignition key turned to run/on, and the defroster switch on, gently touch a good stripe with the test light’s pointy tip, about in the center of the window. You’ll find the light glows dimly, which is a good thing, as that stripe is dropping voltage progressively across its length.

Now you’re ready to go after the other stripes, and you’ll see the difference. Touch the test light gently to each suspect stripe, in various spots across its length. You’ll find the light glows brightly or not at all, depending on which side of the injury you touch. Keep probing until you find the exact spot where the light brightens or dims, and that’s the place to fix.

To make the repair, you’ll apply masking tape to each side of the stripe and apply the repair product, which resembles thick paint, to the injured area. Follow the kit instructions regarding specific application method, cure time and tape removal, and you should be back in business.