In the photo you showed the light is shining directly at the defroster lines, highlighting them. How often does that happen in life? Plus, how thick are those lines? About 0.020" or 0.030" thick? In 1:25 scale that is 0.0012". That is thinner than human hair.
Even if you had a decal made, applying it would leave the clear decal film on the window. That film would not be perfectly crystal clear, which would affect the clarity of the window.
Painting the lines that thin is also out of the question.
Every model I ever worked with that had the defroster lines molded in the "glass" is way oversized. Some (especially from Japanese manufacturers) are quite subtle, but still seem oversize to me. This is one of those details which IMO does not add realism in a model because it is pretty much impossible to scale properly. If the lines molded in the glass on your model look crappy I would go as far as sanding them off and polishing the glass. Cars were also sold with rear windows without the electric defroster lines.