Defogger Doesn't Defog

Have a 2008 Prius still under warranty. Can't find anything wrong except for the rear window defogger. In the 5 months I have owned the car I only needed the defogger two times and it did not work either time. After a half hour driving with the defogger on there was only about 1/8 inch of window defogged at each wire.

The dealer "checked" it and said it checks out but bring it it next time it is happening. It obviously impossible because it happens so infrequently and both times at night.

Has anyone else had a bad rear window defogger?

Any suggestions on getting the dealer to do more than look at it and say it checks out?

 

Thanks for the suggestions. I have multimeter and would like to check whether the element is getting voltage. It might also be possible to disconnect the defogger and do a continuity check for a broken element.

The problem:

Where are the connectors from the defogger to the car wiring located? On my last car they were right on the rear window but in the Prius they are concealed somewhere.

I thought about doing it through the fuse connectors but the fuse box is near inaccessable.

 

I would have thought the window elements would be electrically insulated? But I'll give it a try with the aluminum foil to avoid any element damage.

The defogger was used to remove inside fog...not outside frost. The next time it happens I'll run the front window defogger full blast and see if it works.

Thanks for all the tips.