Useless rear screen defroster

Just recently with the cold spell I've noticed that the rear screen defroster seems almost useless. It takes at least 25-30 mins to clear the rear screen, which is probably clearing from the cars interior heat more than anything else at that point! Even once the switch has timed out, I have to press it again to keep trying to clear the screen. How long should it take for the back screen to clear? It's my first saloon car in many years and I'm really missing the ease a back wiper would clear the screen after the defroster had been on just a minute or so! Is there a problem with mine or do they all take this long?

My S60 does the same. You can see the heating elements working but as quickly as it defrosts, it refreezes. My rear window has been frozen for the past week.

I also find the S60 rear window takes ages to deforst when frosty. Must be part of the design setup.

Seems odd. It's a good job they don't sell these cars in colder countries then... ;)