Rear window demister repair

Most parts stores and possibly some hardware stores should carry a 'repair kit' or a 'pen' for fixing such.

Here is one example of a 'kit' sold in the Americas : https://www.autozone.com/sealants-glues-adhesives-and-tape/window-defogger-repair-kit

And an example of a 'pen' : Defroster Grid Repair Pen - P68551 | LawsonProducts.com

You can also find 'pens' and 'kits' on Amazon and Ebay.

Search for an article or video on how to prep/clean the surface before attempting repair.

 

It would only matter if you wanted to have a working demister/defroster or not

 

You're usually not using it to replace any significant length of the heating trace, but just to paint over a little spot where the trace got cut. Nobody's going to look at it closely enough to notice whether the color matches, and all that really matters is that the paint conducts. (If you were going to replace any significant length of the trace, you'd have to get the resistance and heat transfer capacity right, but that's just not what you're doing when painting over a small break). So, you don't have to stress over it ... it's easy, and works.

-Chap