If you need to reverse your car out of your driveway on a particularly cold morning, you may be used to flicking that switch to clear the back window so you can see where you're going. It's particularly annoying if that switch fails to work, and you'll want to fix the issue as soon as possible, so you don't face similar problems again. What could be wrong with the system in this situation?
Of course, the fuse may have blown. Refer to your manual to locate the fuse in question and replace it as needed.
On to the Matrix
Most modern-day vehicles feature a heating matrix element attached to the inside rear window using a special adhesive. The individual elements run across the screen at given intervals and are, of course, unobtrusive and do not block your vision.
However, as these individual element strips are so diminutive, they can sometimes sustain damage. This is especially the case in hatchback cars, where people may come into close contact with the window itself.
The first thing you should do is check each element to look for signs of any breaks. If you find an issue, you can carefully apply masking tape on each side and then use some conducting paint to fill in the gaps. Find this type of paint in your auto parts shop.
If the matrix appears okay, check the electrical earthing point. Make sure the wires are in good condition and properly secured, and clean any corrosion out of the way if you find it.
Switch and Relay
From this point, you will need to check the switch, relay and wiring in that order. If you have the device, you can use a test lamp, which you should connect to the output terminal of the switch and then the relay. If it fails to light in either case, that particular device will need to be replaced.
Wiring Loom Issues
When the issue is in between the relay and the heater element, this can be more complex. You will need to locate the wire, which will probably be contained in a larger loom and may be covered with insulation. There may be a break in the wiring along this path, which has caused the issue.
The Better Approach
Many people in this situation will simply entrust the work to an auto electrician. After all, it can certainly be "fiddly" and especially difficult if you have to unravel that wiring loom.
For more information on auto electrical work, contact a professional near you.