More importantly if it keeps on misting up could it be something more concerning e.g. rainwater seeping into the car through a dodgy seal somewhere.
My old Astra suffered constant misting of the rear window and I had to always have the rear window heater on to demist. Some time later I had to get the spare wheel out of the boot and when I did I found the cavity in which the wheel is stowed was about 2 inches deep in water. It was a slight crack in one of the hinges of the tailgate that allowed rainwater in over a long time, which dripped inside the boot and collected below the spare wheel. The constant moisture on the back window was from the carpet drying out!
I got the hinge rewelded, thoroughly dried out the carpet and wheelwell, and after that no problem with a constantly misted back window.
Might be worth checking.