There are rear defogger/defroster repair kits available online. But if you can find a conductive pen or any applicable conductive paste, you can DIY.
A quick search provides the following repair steps:
GRID FILAMENT REPAIR
1) Clean broken grid and its surrounding area with a cloth dampened in alcohol. Apply masking tape to both sides of grid line. See Fig. 1.
2) Apply a small amount of silver conductive paint to tip of a drawing pen. Apply silver conductive paint on grid, overlapping existing grid approximately .20" (5 mm).
3) After 10 minutes, check repaired grid for continuity. DO NOT touch repaired area while test is being performed. If repaired area is okay, use a heat gun and apply hot air to repaired area for approximately 20 minutes. If a heat gun is not available, allow repaired area to dry for 24 hours.
Not true. It is also useful the other way around; when the interior is cool enough and there's too much humidity outside. I've used it over a dozen times and even with the A/C set to a comfortable level, the rear window can still fog up under certain situations.