If your vehicle is showing rear window defroster not clearing fog or ice, here are the most likely causes and the parts you'll need to fix each one.
A blown fuse will completely cut power to the rear defroster circuit. This is a built-in safety mechanism that often trips due to age, a minor electrical surge, or a short circuit.
The conductive horizontal lines on the inside of the rear glass can be easily scratched or severed by cargo, aggressive cleaning, or pets. A single break in a line will stop that specific strip from heating.
The relay acts as a heavy-duty switch to send high electrical current to the grid. If its internal contacts burn out, the dashboard button may light up, but no power actually reaches the window.
If the rear window was recently tinted, replaced, or worked around, the electrical tabs may have been left unplugged or the grid lines accidentally sliced by razor blades during the process.
On hatchbacks and SUVs, the wiring harness bends every time the liftgate opens. Over time, the heavy-gauge wire powering the defroster can fatigue and snap inside the rubber accordion boot.
The physical button on the dashboard or the climate control module itself may have failed internally, preventing the system from ever receiving the command to activate.