I don't know if this is unusual or to be expected, but I've always noticed that the rear defroster on my Kizashi performs a bit irregularly... sometimes when I switch it on it will start clearing out my rear window almost immediately, and other times I can drive for miles and miles before I see any effect. The side mirror defrosters on the other hand, always clear things off very quickly.
Is this normal? Or is something broken?
Rear defroster lag
If the density of the moisture on the rear window is different, then it will take longer. If it's just fog, it will clear quickly. If it's heavy dew, it will take much longer. But you probably already know this. The side mirrors since they are vertical will most of the time just have a fog on them so they will clear quickly.
Totally agree.LPSISRL wrote:If the density of the moisture on the rear window is different, then it will take longer. If it's just fog, it will clear quickly. If it's heavy dew, it will take much longer. But you probably already know this. The side mirrors since they are vertical will most of the time just have a fog on them so they will clear quickly.
I also notice that if you clean the inside of your windows regularly, it's less likely to fog up.
2025 Mazda CX-50 Preferred Hybrid
2011 Suzuki Kizashi Sport GTS 6MT (Sold)
2011 Suzuki Kizashi Sport GTS 6MT (Sold)
It will also depend on whether the moisture is on the inside or outside. The heating elements are on the inside so moisture on the inside will clear quickly. Also, it will also be a very thin layer on the inside most of the time. I say most of the time since I don't know how much hot, moist air will be present on the inside due to other occupants! 
You have to also be careful when cleaning the rear window with the defroster from the inside. Too much contact with the elements can also cause problems.