I bought my Mazda used with (luckily) warranty left on it. I noticed that the bottom half of the rear defroster wasn't working. I took the car to the dealership where they determined I needed a new rear window. After receiving 3 broken windows (2 cracked and one with the clip missing) the 4th one was installed. I didn't mind as I had a free rental.

Eventually I had the opportunity to use the defroster and I noticed nothing! It was working whatsoever. This was some light frost on the window and I didn't even start to see line on the window indicating it's working. I brought it back to the dealership and they checked it. Told me everything checked out ok. I thought maybe I was wrong.

I used it once again about a month later and once agian, nothing after 20 minutes. A couple days later, same thing. So I brought it to the dealer again and once agian they are telling me it's checking out fine. Any ideas of what would cause it to seem to work when check by the dealership though in reality it's ineffective?

 

To test it just use a 12 volt test light. Ground the clipped end then gently touch spots across the grid..You can check the entire grid that way and pinpoint a break in it. There should be 2 connectors attached to the grid. One on each side. Maybe they did'nt hook one up and it is getting power but no ground. It would check good that way.